Question : Gear Change During Tranny Rebuild

[NevynPA]

My wife's 99 Grand Prix SE is in need of a transmission rebuild. The car has a 3100 mated to a 4T60-E tranny. I *think* the tranny currently has a final drive of 3.33; but I'm not 100% sure right now (will be checking the tag on it tonight).

I realize this isn't the direction most folks go, but I'm looking to go to a lower ratio for better gas mileage. After talking to Dave at Triple Edge Performance, I found out that I can't do what I really would like, which would be to put a tranny from a 92 DeVille on it (they had a 2.73 FDR).

Would I be able to drop in a 7ASW from a Boneville or LeSabre without issue, or would coming from a 3800 cause problems?

Here's where I was referencing ratios:

http://home.att.net/~subzero350/1997-99_4T60-E.pdf

PDF came from http://www.gmtuners.com/gmtransinfo.htm

I thank all of you who take the time to read this. It's something I'd *LIKE* to do during rebuild, but if it doesn't happen so be it.

[pocket-rocket]

One think I know will be different is you will need your PCM reflashed so the speedo is right. I would imagine it is a direct bolt in because we share the bellhousing bolt pattern with the 3800. I'm not sure if there is a difference in the way the PCM is programmed to control the tranny though.

[3400-95-Modified]

Rebuild the current trans, since you have the better 99 internals, and then just put a different drive and driven gear on it, get the OVER-driven gear ratio, .89:1, or the 37 drive gear and 33 driven gear. That will put you at a 2.92, since I'm guessing the 99 is actually a 3.29 final drive since they didn't use the 3.33 ratio diff much as the years went on from 94.

[NevynPA]

So just swap to 37/33 and re-flash the PCM?

[3400-95-Modified]

Yeah, the PCM you wont need to change as far as speedo is concerned, but you will need to edit the "drive sprocket ratio" setting, AND also you should edit the shift tables so it will shift properly.

Now that I think about it, you really don't need to do the drive sprocket ratio unless you go up in gear... My 96 got mad when the RPM's were higher than they should be and said it was a trans component slip... When I left it as is and went from 3.33 to 3.06 it didn't bother it in the least, but yeah I had to edit the shift tables so it would shift at a reasonable speed/rpm.

[NevynPA]

One think I know will be different is you will need your PCM reflashed so the speedo is right.

Yeah, the PCM you wont need to change as far as speedo is concerned, but you will need to edit the "drive sprocket ratio" setting, AND also you should edit the shift tables so it will shift properly.

Now that I think about it, you really don't need to do the drive sprocket ratio unless you go up in gear... My 96 got mad when the RPM's were higher than they should be and said it was a trans component slip... When I left it as is and went from 3.33 to 3.06 it didn't bother it in the least, but yeah I had to edit the shift tables so it would shift at a reasonable speed/rpm.

Okay, I think I understand what you're saying, but let me double check it.

I shouldn't have any issues with speedo/odo because there is no change to tire size. I'll be turning lower RPM, but that will come through on the VSS, so the speed and mileage won't be off from stock.

Turning lower RPM than expected won't throw a code; turning higher will cause it to throw a slippage code (most likely in my case a P1870).

Did I get that right? Your quotes kind of contradict each other. Pocket-Rocket, maybe what you were saying is only true if I did a swap, and not a rebuild with gear change?

[pocket-rocket]

I actually meant to have the speedo deal edited like what Jon is saying. My mistake and apologies for the confusion. Just ignore my "reflashing" bit. He has way more OBD2 expertise than I do, so I would go on what he is saying.

[gpse3400]

It will work but the speedometer will be off, if you can carefully exchange the tone wheels off the final drives then you wont have to worry much about recalibration. Not for nothing though, that engine is typically flacid on the bottom as it is. Gearing it taller will only put it right out of its usable range of operation and in fact, you'll be down shifting more at cruise than you realize, thus burning more fuel than you are right now. I would not do it, you will be miserable with it. I recommend the same gear box with the same converter; If you are looking for more economy, do a simple tune up, and/ or re tune the computer.