Hi all, new here. Had a few transmission upgrade questions. So here goes. The truck is a 1987 Chevy Silverado that i am fixing up. It's a 2 wheel drive single cab automatic with a 5.7L engine, if that matters. I would like to upgrade my 700R4 transmission to a 6L80e transmission. For those of you who may be unfamiliar with that transmission, it is a 6 speed automatic that GM sells with their high end cars. They can be found in Cadillacs and Corvettes and such. Well, i need the advice of some good transmission guys. I only want to replace the 700R4 if it will actually help me in terms of highway miles per gallon. Well maybe not boost me up, but at least keep me at the same MPG that i get now at highway speeds. See, right now i have 2:73's in the rear end of the truck. Well, i want to be able to put in some 3:73's, (or possibly even 4:10's) but still be able to do 75 mph on the highway at 1,950 RPM like i can now. The only way i can see to do this would be to have more gears in the transmission. However, if you look below, you can see that all in all, there is only a .03 difference in the last gear on the 6L80e transmission.

6L80e gear ratios:

1st - 4.02
2nd - 2.36
3rd - 1.53
4th - 1.15
5th - .085
6th - 0.67

700R4 gear ratios:

1st - 3.06
2nd - 1.63
3rd - 1.00
4th - .070

That does not look like it would make enough difference over the 700R4 to justify the expense of the transmission swap. Or would it? Maybe i am wrong here for some reason. I have to admit that i am a transmission noob. So maybe you could help me out and tell me if i could accomplish my goal with this transmission. Again, my goal is to be able to do highway speeds at low RPM (1900-2100) with 3:73's or possibly even 4:10's in the rear end. Think swapping the 700R4 to a 6l80e would do it?

 

Logically, yeah it would work perfect for the idea you are seeking. But I dont know how much more you would have to do. Of course you would have to run a full computer set up on the drive train, I think. More gears, more electronics. These transmissions shift according to sensors on the engine. The MAF sensor, TPS, IAT, all of those give feedback to the computer to run the whole drive train accordingly. Who knows how much more sensors with the new 6l80e. So I would do more research on this. So my answer would be heck yeah, go for it, if its do able without so much headache.

 

Just like Ruffneck was saying: the electronics and sensors and such would be something to consider. But aside from that, I'm not so sure that you would run the same rpms at highway speeds if you also installed a 3:73 or 4:10 differential. It will probably be a little bit higher rpm but overall mpg I believe would go up. I think this because with more gears closer together would cause less load on the engine to move up to driving speed. The city mpg I believe would go up significantly, but going on a long highway drive wouldn't be so. I am all up for putting in a six speed transmission.

 

are the improvements you will see worth the trouble and modifications that are needed? Probably not. going with a 4L60E or 65E is easier than the physiacal mods needed for the 6L80E. Its doable but if this is a daily driver it is not worth the trouble.

 

Yeah good point. Anyone know that formula that they have... it goes something like if you have a certain tire size, and you run a certain rear end, then you will be running "X" rpm at "X" speed. I would like to plop some 3:73's in the rear end but i still want to be able to do 75mph at a decently low RPM for engine longevity and fuel mileage reasons.

 

3.73'S are still gonna be ok. Its the threshold but its the most common choice for power highway ratio.

 

I vote for 3.73's and keep the 700R4. It's not a bad combination at all. You might turn a few hundred more RPM's at cruising speed, but not enough to get annoying. Now my old blazer that had a 700R4 with 4.10's was fun around town, but wasn't a great highway vehicle at all. Like it has already been said here 3.73's are about the perfect compromise. I think the 700R4 is one of the best transmissions made. I have seen them hold a ton of HP.