Rear-end gearing with auto trans

[K-Loader]

What are your thoughts on the two different rear-end gear ratios available with the GT auto-trans, i.e. 3.15 vs. 3.55? I ordered the default 3.15 gear ratio. I’m now second guessing my decision. The car will be my daily driver, averaging two hours a day round trip between home and work, with a high speed no traffic drive to work, and a low speed heavy traffic return trip. What gear ratio would you prefer?

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[BmacIL]

For that usage, 3.15 for sure. Lower cruise RPM = better FE, lower cabin noise.

 

[foghat]

3.15 will get you better mileage on the highway.

I'd prefer the 3.55s as they will offer peppier performance

 

[Five Oh Brian]

3.15 will get you better mileage on the highway.

I'd prefer the 3.55s as they will offer peppier performance

True, and I agree. Although, I'd like to add that 4.10's are incredibly entertaining with the 6R80 transmission.

 

[Mustang8]

Isn't the FE between 3.15 and 3.55 pretty small?

 

[Five Oh Brian]

Isn't the FE between 3.15 and 3.55 pretty small?

In town mileage is actually a bit better with 3.55's as you need less throttle to move the car from a stop. Highway mileage drops about 1 mpg with 3.55's as the engine is turning a little faster on the highway. For someone who drives mostly in town, the 3.55's will actually yield better overall mileage. For someone who is primarily on the highway, the 3.15's are a better choice for mileage. For someone who lives a 1/4 mile at a time (like me), it's 4.10's FTW.

 

[K-Loader]

Thanks for the feedback everyone. I found some 20-year old test data on a Nova comparing a 3.00:1 and 3.50:1 axle ratio. The data showed a .35 sec difference in the 1/4 mile ET and 300 RPM difference at 60 MPH.

 

[1320']

One of our members has a Auto 3.55 geared GT and he showed a shot of his HWY MPG right at 26.15 or something right about that. The 26 MPG rating obviously now was done with 3.55's

 

[Old 5 Oh]

For that usage, 3.15 for sure. Lower cruise RPM = better FE, lower cabin noise.

^^ This. I made the same decision for the same reasons.

 

[65fastback2+2]

with the 6R80 having a 4.17 first gear...either is going to be fine.

3.15*4.17= 13.1355

To compare with the old new edge 4spd auto

4.56*2.84=12.9504

So the 6R80 with 3.15 rear gears has a better first gear ratio than the old 4R70's with a 4.56 rear gear. Phenomenal.

 

[Fomoco15]

Bout the same deal with me. I went with 3.55.

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