Fuel Mileage Question

[perfimage]

Just a question about fuel mileage in my 93 z34.

146k miles, no mods, just straight stock, Timimg belt replaced about 30K ago. Engine runs perfect and strong.

My normal fuel mileage for the last 2 years has been roughly 17.5-19.5 MPG, and sometimes as low as 16.8. Now mind you, this is my daily driver and I runt eh exact sme rout to work every day at the same times of the day, so my mileage numbers should not vary much at all.

To do my last oil change, my wife and daughter picked up my supplies while shopping and I ended up with SPT Gas Treatment instead of Oil Treatment. So I put the Gas Treatment in at my next fill up.

To my surprise, I saw an increase in my mleage of 10-15% on that tank. I filled up again with about 1/8 tank left, but did not add any Gas treatment and that tank saw similar numbers. The next 2 tanks saw numbers back down where they had been (17.5-19.5). On the next tank I tossed in a bottle of STP Gas Treatment and saw the increased mileage.

Here are the actual numbers for my last 7 tanks:

19.41 mpg..No Gas Treatment
19.16 mpg..No Gas Treatment
18.98 mpg..No Gas Treatment
21.40 mpg..Gas Treatment added at this tank
22.48 mpg..No gas treatment added, but tank filled at about 1/8-1/4
18.41 mpg..No Gas Treatment
20.94 mpg..Gas treatment added at this tank.

Now this seems awfully odd and I thougt it was just a fluke until I added the Treatment the second time.

My question is, is there any explanation for the DRASTIC differences in fuel mileage? I really doubt the Gas Treatment alone can have that big an affect on mileage, but the numbers are impressive.

My thought is this. Is it possible that I have an O2 sensor failing or reading wierd and the addition of the Gas treatment changes the exhaust gases in such a way that the O2 sensor reads a little "rich", causing the computer to lean the mixture out and improve mileage.

I am not seeing any SES light at all and no codes are stored.

Any thoughts?

RED

[RedZMonte]

not sure what the reason the additive is increasing mileage. maybe its boosting the octane....

i'm getting 20-24mpg even after i went turbo, depending how i drive it. a few maintenance items i would do to ensure everything is up to par.
* Tune up (new plugs (ACDelco/NGK), PCV, Clean throttle plate, GM Top engine cleaner, Air filter)
* Plug wires
* Fuel filter
* BG 44k (No substitues, USE BG... Fuel injection cleaner)
* BG MOA (Oil treatment)

if the O2 sensor is getting really old it may be time to replace it... It does tell the ECU what fuel mixture your running... it could be getting wore out. I would definaly get a OEM style not a "Crimp On" generic O2.