fuel related topics

[IanSzgatti]

wouldn't a category devoted specifically to fuel and fuel additives/ nitrous/ be handy here? There is a LOT to know about gas!!

[bszopi]

Nitrous is covered under the boost section of the site. All other additives can be covered either in off-topic, maintenance or performance areas of the site. We have a Track Talk section that hardly gets used... I really don't think a section on fuel would get used much at all. Perhaps initially, but after awhile, I just don't see any new topics being discussed. Sure, there is alot to know about fuel, but I just don't see members talking about it enough that it requires a specific section for it.

[SappySE107]

Perhaps he meant on the site, not the forum. I think we need to update all of the site but its finding intelligent people with time to cover the topics that we are lacking.

[IanSzgatti]

but after awhile, I just don't see any new topics being discussed. Sure, there is alot to know about fuel, but I just don't see members talking about it enough that it requires a specific section for it.

<--- over to that side in the nav bar, where the myths setion is. Im sure you guys know about fuel, octane, alcohol, etc. I just found some of those articles pretty facinating, hence the suggestion.. I would be willing to put together some information if needed.

[bszopi]

Ok, that sounds fine, especially if you want to pull the information together. Type up some stuff and send it over and I'll get it up on the site.

[IanSzgatti]

Gasoline

Gasoline is a distilled and refined oil product made up of hydrogen and carbons - a hydrocarbon. A long-chain hydrocarbon to be exact (so don't get it on your skin - its carcinogenic). It's designed to be relatively safe to handle, if you're careful. ie. it doesn't spontaneously combust without extreme provocation. When you have a petrol fire, it's not the petrol itself that is burning, it's the vapour, and this is
the key to fueling an engine. The carburettor or fuel injectors spray petrol into an air stream. The tiny particles of petrol evaporate into a vapour extremely quickly, and combined in a cloud with the air, it becomes extremely combustible. The smaller the particles from the carburettor jet or fuel injector, the more efficiently the mixture burns.

Detonation, pre-ignition, pinking, pinging and knocking.

Remember I said petrol doesn't spontaneously combust? Well it can if the conditions are right, and the conditions are extreme heat and pressure - exactly the conditions you find in the combustion chamber. When this happens, it's called detonation or pre-ignition. Diesel engines rely on this process because they don't have a spark plug in the traditional sense of the word. However in petrol engines, when this happens (also known as dieseling), it's a Very Bad Thing. Engines are designed to have the fuel-air mix burn at a fixed point in the cycle, not explode randomly. Whilst it might look like an explosion, if you could film it on a super high-speed camera, you'd see the mixture actually burns up very quickly rather than exploding. Detonation, dieseling or pre-ignition are all terms for what happens when the fuel-air mix spontaneously explodes rather than burning. Normally this happens when the mixture is all fouled up, and the engine is running hot. The temperature and pressure build up too quickly in the combustion chamber and before the piston can reach the top of its travel, the mixture explodes. This explosion tries to counteract the advancing piston and puts an enormous amount of stress on the piston, the cylinder walls and the connecting rod. From the outside of the engine, you'll hear it as a knocking or pinging sound. The precise sound is very hard to describe because every engine sounds slightly different when it happens. But the best way I can describe it is a constant 'toc toc toc' type knocking sound.

Compression ratio.

The compression ratio of an engine is the measurement of the ratio between the combined volume of a cylinder and a combustion chamber when the piston is at the bottom of its stroke, and the same volume when the it's at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Similarly, the higher the compression ratio, the greater the liklihood of detonation.

Octane ratings - how to stop detonation

So you know that a fuel-air mix, given the right conditions, can spontaneously combust. In order to control this property, all petrols have chemicals mixed in with them to control how quickly the fuel burns. This is known as the octane rating of the fuel. The higher the rating, the slower and more controlled the fuel burns.
Put on the geek-shades for a moment and I'll explain octane in more depth. If you don't like being blinded by science, skip down a few paragraphs. For the rest of you, octane is measured relative to a mixture of isooctane (2,2,4-trimethylpentane, an isomer of octane) and n-heptane. An 87-octane gasoline has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane. The octane value of a fuel used to be controlled by the amount of tetraethyl lead in it, but in the 70s and 80s when it became apparent that lead was pretty harmful, lead-free petrol appeared and other substances were introduced to control octane instead.

Measuring octane - RON, MON and the difference between America and the rest of the world.

Just so you know, the octane number is actually an imprecise measure of the maximum compression ratio at which a particular fuel can be burned in an engine without detonation. There are actually two numbers - RON (Research octane number) and MON (Motor Octane Number). The RON simulates fuel performance under low severity engine operation. The MON simulates more severe operation that might be incurred at high speed or high load and can be as much as 10 points lower than the RON. In Europe, what you'll see on the petrol pumps is the RON. However, in America, what you'll see on the petrol pump is usually the "mean" octane number - notified as (R+M)/2 - the average of both the RON and MON. This is why there is an apparent discrepancy between the octane values of petrol in America versus the rest of the world. Euro95 unleaded in Europe is 95 octane but it's the equivalent of American (R+M)/2 89 octane.
In America, low altitude petrol stations typically sell three grades of petrol with octane ratings of 87, 89 and 91. High altitude stations typically also sell three grades, but with lower values - 85, 87 and 89.

What factors affect detonation?

There's a bunch of things that can affect how likely an engine is to have detonation problems. The common ones are ambient air temperature, humidity, altitude, your engine's ability to stay cool (ie. the cooling system) and spark timing. Fortunately, nowadays the engine management system of modern cars can compensate for almost all of these by advancing and retarding the ignition timing. This is where the computer slightly adjusts the point in the ignition cycle at which the spark is generated at the spark plug. With older engines that used mechanical points to send current to the spark plugs, adjusting the timing was a manual affair that involved adjusting the distributor cap orientation.
Knock sensors. Most modern cars have knock sensors screwed into the engine at multiple places. These actually detect the vibration or shock caused by detonation (rather than trying to detect the sound) and can signal the engine management system to change the ignition timing to reduce or eliminate the problem.

Octane and altitude

The higher the altitude above sea level, the lower the octane requirement. As a general rule of thumb, for every 300m or 1000ft above sea level, the RON value can go down by about 0.5. For example an 85 octane fuel in Denver will have about the same characteristics as an 87 octane fuel on the coast in Los Angeles. As a practical example of this, I currently live in Salt Lake City which is at around 4,200ft. We travel to Las Vegas from time to time which is at around 2,000ft. Our Subaru has a minimum octane requirement of 89 at sea level - so about 87 where we live. Last time we drove to Vegas, the petrol station we stopped at had run out of 'premium' products so we had to fill up with 85 octane. This, combined with the drop in altitude caused the 'check engine' light to come on because we'd effectively taken the engine from 87 octane at altitude to the equivalent of 83 octane at altitude - way below the minimum required by our car.

Octane and power

It's a common misconception amongst car enthusiasts that higher octane = more power. This is simply not true. The myth arose because of sportier vehicles requiring higher octane fuels. Without understanding why, a certain section of the car subculture decided that this was because higher octane petrol meant higher power.
The reality of the situation is a little different. Power is limited by the maximum amount of fuel-air mixture that can be jammed into the combustion chamber. Because high performance engines operate with high compression ratios they are more likely to suffer from detonation and so to compensate, they need a higher octane fuel to control the burn. So yes, sports cars do need high octane fuel, but it's not because the octane rating is somehow giving more power. It's because it's required because the engine develops more power because of its design.
There is a direct correlation between the compression ratio of an engine and its fuel octane requirements. The following table is a rough guide to octane values per engine compression ratio for a carburettor engine without engine management. For modern fuel-injected cars with advanced engine management systems, these values are lowered by about 5 to 7 points.

Compression ratio Octane
5:1 72
6:1 81
7:1 87
8:1 92
9:1 96
10:1 100
11:1 104
12:1 108

Octane and gas mileage

Here's a good question : can octane affect gas mileage. The short answer is absolutely, yes it can, but not for the reasons you might think. The octane value of a fuel itself has nothing to do with how much potential energy the fuel has, or how cleanly or efficiently it burns. All it does is control the burn. However, if you're running with a petrol that isn't the octane rating recommended for your car, you could lose gas mileage. Why? Lets say your manufacturers handbook recommends that you run 87 octane fuel in your car but you fill it with 85 instead, trying to save some money on filling up. Your car will still work just fine because the engine management system will be detecting knock and retarding the ignition timing to compensate. And that's the key. By changing the ignition timing, you could be losing efficiency in the engine, which could translate into worse gas mileage. Again as a practical example, my little tale above about our trip to Vegas on low octane gas. (Whether you want to believe some bloke on the internet or not is up to you). On the low octane gas on the trip down, we could barely get 23.5mpg out of the Subaru. Once I was able to fill it up again with premium at the recommended octane rating, we got 27.9mpg on the way back. A difference of 4.4mpg over 450 miles of driving.
Doing the maths, you can figure out that by skimping on the price during fill-up, you may save a little money right there and then, but it costs in the long term because you're going to be filling up more often to do the same mileage. My advice? Do what the handbook tells you. After all it's in the manufacturers better interests that you get the most performance out of your car as you can - they don't want you badmouthing them, and in this day and age of instant internet gratification, you can bad-mouth a large company very quickly and get a lot of publicity.

Octane boosters

In some extreme cases, the highest octane fuel available might not solve a knocking or detonation problem. That's normally a symptom of a deeper problem in the engine involving carbon deposits on the cylinder heads, bad spark timing, faulty engine management systems or similar. In these cases, some people choose to add octane booster to their petrol. Basically you fill the tank as normal, then put in a measured amount of octane booster and it further raises the octane level in an attempt to stop the detonation. One of the downsides of this is that it can make the engine harder to start from cold, because the octane booster has made the fuel so much less volatile that it's hard to get it to ignite on the first couple of strokes. Products like Klotz and Redex octane boosters are readily available over the counter in most auto parts stores. Octane boosters are typically used by mis-educated motorcyclists who believe the myth (explained above) that high octane = more power.
In England, octane boosters are typically also sold as "lead replacements" or "4 star additive". A lot of European cars relied on the lead in 4-star petrol for the increased octane. Lower octane unleaded fuels caused a lot of problems when they first appeared, especially with cars that didn't have engine management systems. Knocking and detonation became evident in a lot of cars and for some reason French and German engines were more susceptible than most. Dumping a shot of octane booster in the tank when filling up solved the problem by raising the RON a few points to make it the equivalent of what old leaded petrol had been. Eventually, by the late 90s, most English and European petrol stations introduced LRP - lead replacement petrol, and the problem went away. Well. Sort of......

Lead Replacement Petrol (LRP) and valve seats

Whilst LRP solved the problem of lower octane unleaded petrol, it introduced a new problem. The lead in leaded petrol also had a secondary function and that was to lubricate the valve seats - the top of the engine block where the valves "park" when not being opened by the cams. With the advent of LRP, detonation went away but the chemicals used to increase octane didn't have any lubricating function. Some older engines started to suffer from increased wear to the valve seats, to the point where the valves could no longer properly close and seal the intake and exhaust ports. There were a couple of high profile cases before I left for America in 2001 but I've never been able to find out the end result. If you have any information on what happened in these cases, drop me a line and I'll include the info here.

The supermarket petrol debate

tesco petrol stationDuring the 90s, in England, supermarkets started a price war with the mainstream fuel vendors by opening their own petrol stations and undercutting the Esso's and Shell's of the world by as much as 5%. People flocked to these cheap outlets without doing any proper research and after a couple of years, a lot of vehicles began to suffer as a result. There's an old saying that begins "if it's too good to be true....." In the case of supermarket petrol, there was an obvious reason why it was cheaper - it was the lower grade fuel that the mainstream outlets wouldn't take. Stuff which had been rejected in quality control, or had less additives and detergents than what you might get from Texaco or Philips66. As a result, engines started clogging up and failing emissions test. Gas mileage went down. Engines became lumpy and rough running and eventually the supermarkets were forced to fall in line with the Big Boys, so much so that nowadays they're normally less than 1% cheaper.
Skip forwards to 2005 and the summer of high fuel prices in America. Lo and behold, supermarkets started to sprout petrol stations and at the time of writing, a lot of people are in the same "cheap fuel" euphoria that the English were in 10 years ago. Sooner or later there'll be a high profile lawsuit that will put paid to that though.
costco petrol stationAs a substitute for genuinely cheaper fuel, a lot of European supermarket chains now offer cheaper fuel at a price. The catch is that you have to shop with them. Once you buy a certain amount of stuff from their store, they'll knock off a percentage of the price of petrol if you buy it from them. The fuel isn't the cheap and nasty sub-standard stuff of yesteryear that they used to use - it's good, mainstream product. But they can hide the price drop in the cost of the groceries and other items you buy in store. From your perspective, you save £2 a tank when filling up. From the store's perspective, you just spent £100 in shopping so giving you £2 back on your tank of gas is pocket change.
In America, some of the big-box chains, like CostCo and Sam's Club are now doing the same thing. Rather than go the "dodgy crappy petrol" route, they're offering discounted petrol for shopping in their stores, discounting the petrol by a couple of cents per gallon as long as you've bought more than $50 of products from them.

E85 Ethanol - the magic bullet?

With the spiralling cost of fuel prices brought on by George Bush's "War On Terror", people are looking at everything to get cheaper fuel, and one of the silver bullets seems to be E85 Ethanol-blend gasoline. I say 'seems to be' because once you do some research, which is what you're doing right here by reading this, you'll learn it's not quite the magic solution everyone would have you believe.
E85 ethanolE85 is a blend of 15% regular unleaded petrol with 85% ethanol. It's designed for so-called Flexible Fuel vehicles, and as such has been classified by the US Department of Energy as an alternative fuel. The facts on E85 are a little hard to come by, so I've put as many as I can right here so that you, dear reader, can try to cut to the chase. So what is a flexible fuel vehicle (FFV)? Well, it's a vehicle with an engine and emissions system designed to be able to run on a blend of unleaded petrol and ethanol up to a maximum of 85% ethanol. If E85 isn't available, you can run them on just plain old petrol though. If you read all the hoopla surrounding E85, you'll see this statement crop up time and time again: "It is a renewable source of energy and reduces the crude oil imports needed to fuel America’s transportation system. Ethanol is a clean, environmentally friendly fuel.". Weeeeelllllll yes. But more importantly, no. It's true that it is partly based on a renewable source of energy - ethanol is basically distilled corn oil (or wheat, barley, or potatoes. Brazil, the world's largest ethanol producer, makes the fuel from sugarcane), and yes, it's a cleaner and slightly more environmentally friendly fuel. There's a 'but' to go with all this, and it's a big 'but' - of Jennifer Lopez proportions. Here it is. In bold because it's the important part of this paragraph. E85 Ethanol-blend fuel has horrible gas mileage.
What does this mean to you? Well it means you'll need a lot more of it for a start. Sure it may be cheaper than regular petrol, but there's a reason - it's a terrible way to run a vehicle. Even the governments own figures back that statement up. Check out one of their lists of flexible-fueled vehicles for yourself. On average, putting E85 in a flexible fuel vehicle will return a whopping 25% worse gas mileage. E85 doesn't burn as efficiently as regular petrol because it contains less energy per volume - 75,760btu per gallon as oppose to 115,400btu per gallon for plain old petrol. This accounts for the 30% increase in the amount of fuel required in the fuel-air mix during combustion, and the corresponding drop in gas mileage. All this comes with an average drop of only 10% in greenhouse gas emissions. If you go by historical precedent, and assume we all move to FFV's, the income from regular petrol will drop so the oil companies will simply increase the cost of E85. At that point, you're getting terrible gas mileage but paying what you used to for just plain vanilla unleaded petrol. Remember - nothing is free.

But what about Brazil?

For a while now, Brazil (the country, not the Terry Gilliam film) has managed to be largely independent of the world's fluctuating oil prices. By law, all Brazillian petrol must be at least 25% ethanol - E25 - created from sugar-cane-fed biorefineries. By 2007, almost all cars available in Brazil ought to be able to run on 100% ethanol. (It's worth noting that Ethanol-only cars were sold in Brazil in significant numbers between 1980 and 1995). No longer dictated-to by Big Oil, the price of their E100 is relatively low and thus it offsets the lower gas-mileage quite nicely. One argument put forth in America is that using E85 will reduce the reliance on foreign imports - specifically oil. But you need to look at the whole picture. E85 comes from corn, currently a crop used to feed people. Assuming that America has enough spare capacity to farm corn for E85 for the current demand, what happens when more people start using it? You can't increase farmland, or drop production of corn for food, so the next alternative is importing it. At which point, even using E85, you become dependent on foreign imports again. Brazil doesn't have this problem because their system is in balance and so they supply themselves with enough surplus to export their product. Most likely to America.

Clean exhaust - it depends on your definition of the word "clean".
ch3cho

Something that isn't widely publicised is the difference in emissions between corn-based ethanol, as used in America, and sugar-based ethanol, as used in Brazil. We're all told that ethanol blend fuels produce cleaner exhaust and with sugar-based ethanol, that's absolutely true. Even with corn-based ethanol, the gasses measured at an emissions check are lower, which still looks good. But there are two things a corn-based ethanol E85 vehicle will produce through the exhaust that might surprise you. First, for the same size of engine and weight of vehicle, a corn-based E85 engine dumps nearly double the CO2 into the atmosphere. And second, the exhaust gas contains acetaldehyde (CH3CHO) and lots of it. Acetaldehyde is a known carcinogen and suspected neurotoxin, and when exposed to its vapors, you or I would likely develop irritation of the eyes, skin and the respiratory tract. In fact, Acetaldehyde is ranked as one of the most hazardous compounds (worst 10%) to ecosystems and human health. It's obvious why this isn't widely publicised, but then you might ask the question "why don't we see this in the emissions test?". Simple. The emissions test doesn't look for it. You can't detect and measure something you're not looking for.
But wait - it gets better. The corn-based ethanol production process consumes more fossil fuel energy than ethanol's actual calorific value. In other words, to produce a gallon of ethanol to be used in E85, it takes more fossil fuel energy than you could simply get by putting a gallon of refined non-blend petrol in your car. And as you know, regular petrol also gives better economy.
E85 in non-flexible fuel vehicles

Two words : rotting seals. And I'm not talking about dead sealife. E85 is pretty acidic, and stuffing it in your regular petrol-engined car will do no end of damage to it. Apart from the spark timing and the fuel-air ratio being totally different, E85 has a whopping 105 octane rating to deal with the pre-igntion problems of having 30% more fuel in the fuel-air mix during combustion. The seals and gaskets in FFVs are designed to withstand the acidic deposits that E85 generates during combustion. Generally speaking, FFVs are manufactured to eliminate all bare aluminium, rubber and magnesium parts - all items which E85 is known to rot, and all items which a normal engine has by the bucketload. Another problem with E85 is that it's electrically conductive. Regular petrol fuel pumps aren't really designed to work with a conductive fuel, so using E85 with one could result in a fuel fire where the petrol is not only the fuel for the fire, but the electrical path for the spark. FFV fuel lines aren't made of rubber, but typically stainless steel lined with plastic.
So you may get cheaper petrol but you'll get worse gas-mileage and a broken car, with the possible bonus of a raging E85-fueled inferno to boot. However, there is a fly in the ointment here, and it is E10. Because of petrol company practices (see below), most fuel-injected engines designed and built since 1988 are already somewhat adapted to using ethanol, just not in the percentage you find in E85.

E10: you're using it right now

It's not widely known but a lot of petrol companies now blend 10% ethanol into their petrol products without really admitting to it, much less advertising the fact. If you've noticed your car runs somewhat less than the advertised gas mileage, that's part of the reason. So in America, if the EPA tells you you should be getting 20mpg city and you're only getting 18mpg, even driving with a feather right foot, it's not you, it's the petrol companies. 10% ethanol blend will rob you of about 5% gas mileage, and EPA figures assume a pure non-ethanol petrol. The reason for doing this is simple : money. Petrol companies can sell 10% blend without advertising the ethanol content and charge full-price for it whilst it costs them a lot less to produce. So until some hotshot lawyer in America files a class-action suit, we're all driving around with a more acidic petrol mix in our engines, which is contributing (somewhat) to premature engine wear. You can bet your bottom dollar (or euro) that the European refineries are doing exactly the same thing.

Tax credits, subsidies and tariffs - the real story behind E85.

So given the (obscured) facts about corn-based ethanol, why the big push in America to go to E85? Simple. Money. The government is offering tax credits to the big car manufacturers to produce FFVs, even if none of them ever run on E85. Similarly, tax credits are now offered to the big oil companies to product E85 ethanol blend, even if they don't actually sell it. And when they do sell it, it will make the more money because you and I will need more of it to go the same distance. Finally, corn growers receive federal subsidies for growing corn for ethanol production. Couple that with the 54¢ per gallon tariff that is currently levied on Brazillian imports, and it shows how the corn-based ethanol has cornered the American market and is keeping the cheaper, cleaner, sugar-cane ethanol at bay.

What's that you say? You want fuel efficiency and less cost?

VW Polo BluemotionWell that's the conundrum isn't it? The oil and car companies aren't going to give stuff away free. So you have to choose. Do you want less cost at the point where you're putting the petrol into your car, or less cost-of-ownership? It's like comparing financial planning. If you get a flexible fuel vehicle, your immediate cost is much less - you could be spending 30% less per fill-up. But the long-term costs become negligible because of the bad gas mileage. On the other hand, if you take the long term investment point of view, you should be looking at vehicles like the VW Polo Bluemotion. It's a three-cylinder turbodiesel, which means at the point of filling it up, you'll actually be spending more than a regular petrol vehicle. But it returns 70mpg (max), so you'll be visiting the petrol station a lot less frequently. Don't understand the maths? Ok, lets lay it out.
I'm going to assume plain fuel costs here, so I'm not factoring in insurance, wear and tear, initial cost of the vehicle etc. Ready? Okay, we're going to compare two vehicles. Each drives 15,000 miles a year and each has a 16 gallon fuel tank. The owners of these vehicles are Barbie and Ken, and to be suitably sexist, Barbie has a pink VW Polo and Ken has a blue Ford Crown Victoria. They both fill up when the tank gets to 3 gallons left, so they drive 13 gallons at a time.
kenKen
15,000 miles = 1250 gallons at 12mpg on E85.
1250 gallons = about $2300 assuming about $1.85 a gallon.
Ken stops and fills up every 156 miles.

barbieBarbie
15,000 miles = 225 gallons at 66.5mpg (split the values of 60mpg and 73mpg for city and highway) on diesel.
225 gallons = about $787 assuming about $3.50 a gallon.
Barbie stops and fills up every 864 miles.

So whilst Ken pays much less each time he fills up, he's filling up nearly 6 times as often, and at the end of the year, he's spent a whopping $1500 more in fuel costs on this nice, 'clean, environmentally-friendly' E85 ethanol. Now I don't know about you, but it seems to me that the pollution from 225 gallons of diesel is going to be a whole hell of a lot less than the pollution from 1250 gallons of E85.
Obviously this example is extreme, but it does use real-world facts and figures from real-world vehicles you can buy right now. I did it to illustrate how being in posession of the facts can help clear up the doublespeak and misinformation. So if you're considering an E85-fuelled vehicle, you might want to do some more homework first, because it most certainly is not the silver bullet we're all being led to believe.
For more information / propaganda, go to the official E85 fuel site.


Gas-mileage, mpg and why American cars can never match the EPA estimates.

Gas-mileage is the quickest indicator of how efficient a car is in terms of fuel used for distance driven. Engine size and power, driving conditions, weather (wind especially) and vehicle weight all affect mpg. Measuring gas-mileage is really easy but it's surprising how many people don't know how to do it. Basically, zero your trip counter next time you fill up, then drive as normal. When you fill up again, let the petrol pump fill to the auto-cutoff point and then make a note of the trip meter reading. Gas mileage is the number of miles on your trip meter divided by the number of gallons the petrol pump put into your tank. You'd be surprised the number of people who use the manufacturer figure for the size of the tank in that calculation instead of the amount of petrol actually put in.
In England and Europe, pumps deliver in litres, so in the UK it's miles-per-litre, although most advertising still uses miles per gallon. It's worth noting that an English gallon is 1.2 US gallons. So when you see a car in England that advertises 40mpg, it's the equivalent of 33mpg in the US.
In the rest of Europe it's normally advertised as litres per 100 km. So for example, 28mpg (UK) is about 10litres/100km. Often this is short-handed to 1-in-10, meaning 1 litre used in 10km of driving.
The EPA

The American EPA (Environmental Protection Agency) rates all cars sold in America with gas-mileage figures, advertised as EPA-rated mpg figures on the new car sticker. It's one of the things car manufacturers rely on to sell their vehicle, especially with today's high fuel prices. Not many people understand this, so I'm here to take some of that confusion away and tell you what the EPA figures really mean.
epa stickerFirst of all, there's the sticker you'll see in every new car in an American showroom, an example of which is seen on the right. There's a load of technical blurb on there to advertise the vehicle, but the two big numbers are the EPA-certified fuel information figures. In this case 20mpg city and 28mpg highway. So you see these figures and you get into your head a rough idea of how often you'll be filling up. The problem is that these are very rough estimates. If you read the small print, it says this:
"Actual mileage will vary with options, driving conditions, driving habits and vehicle condition. Results reported to EPA indicate that the majority of vehicle with these estimates will achieve between 17 and 23mpg in the city and between 23 and 33mpg on the highway."
Okay so it's pretty obvious that driving habits, conditions and vehicle options (like a bloody big roof rack) will affect your mpg, but what's less obvious is the "between" figures. It's basically a get-out clause. In this example, the vehicle is more likely to get 17mpg in the city and 23mpg on the motorway - the low end of the "between" figures.
In the 1980s, the EPA conducted a study on their results vs. the real world, and discovered most drivers got significantly lower mpg figures than the EPA predicted. As a result, EPA estimates on the new car labels were dropped by 10% for city and 22% for highway from their actual results. In 2006 they dropped another 8% from those figures again to try to make the numbers match more closely.
Even that isn't the end of the story though. What you really need to know is how the EPA come up with the figures in the first place. Before you carry on, you might want to put down any drinks or breakables because I know what your reaction will be at the end of this. Ready?
Congress and car company lobbyists require the EPA to measure mpg figures using the following simulated real world conditions in a lab. That's right - EPA testing happens on a dyno in a lab, not on the open road.

* Average highway speed : 49mph
* Maximum highway speed : 60mph
* Temperature : 75°F
* No rapid acceleration
* No air conditioning
* No passengers
* No rough roads
* No hills
* No wind
* No low tyre pressures
* No ethanol in gas

Well the first problem is the last point : no ethanol in gas. In America, you can't buy zero-ethanol petrol - it's all E-10 (see above) so you're already going to be down 5% on the EPA figures even if you could meet all the other requirements. And for the love of God, who drives like this? 49mph on the motorway? Maximum speed 60mph? Perhaps when the model-T Ford was the Big Thing, these were valid speeds, but nowadays (and by 'nowadays' I mean 'in the last 6 decades') motorway speeds are typically 70mph maxing out at 90mph (if you're in Europe anyway). What about the rest of it - no hills, no passengers, no rough roads? Have the EPA actually driven a vehicle in the real world recently?
As a rough benchmark, driving at 65mph instead of 49mph will decrease mpg by 20%. Driving at 75mph will take another 25% off that. In short, you should pay very little attention to the EPA estimates because they are, for the most part, completely meaningless.
Muddying the waters even further
Remember above I said that the city and highway figures were "between" figures, or the average of the high and low EPA tests? Well I'll give you one guess which figure the car manufacturers use in their print and media advertising. That's right - the high-end of the range. In the example above, the low highway figure was 23mpg and the high was 33mpg. In this case, the advertising will always publicise the 33mpg figure. You will likely get not much more than the low city figure - 17mpg.

[SappySE107]

Im gonna guess you didn't type that all out.

[IanSzgatti]

You guessed correctly. I know my gas, but I didn't wip up this piece. If it has to be an original work I can put in more effort, but this is a great article covering may fuel related topics I thought would add a little more valuable information to this already informative website. I don't mind at all if it's not useful though, just a suggestion. Thanks

[bszopi]

I don't want to publish it without a link to the original site. If you can provide that, then I'll put it up.

[IanSzgatti]

http://www.carbibles.com/fuel_engine_bible.html

[bszopi]

I still have some editting to do, but...

http://60degreev6.com/showthread.php?t=34868

Its located in the General Info section.

[SappySE107]

Wow, that looks really nice