blake79

I have a 2013 Mustang GT automatic and that I am looking to change the stock gearing 3.15's to something like a 3.31 or 3.55. However, I also drive on the highway from time to time for long trips. My question is, cruising around 70-75 MPH, what RPM am I sitting at in both ratios? Right now, around 78 MPH I sit at 2,000 RPM. I'm really looking for some extra grunt, but also dont want to kill my highway mileage. Thanks!

howarmat

3.31 wouldnt be worth the cash to do the upgrade. 3.55 might not even be. 3.73 is a sweet spot IMO.

i would guess around 2300-2500ish for the rpms with 3.73

blake79

I agree but this is my daily. I'm worried 3.73 would be too much for highway travel.

howarmat

3.55 will probably get you buy fine then. I know that 3k rpms and a loud exhaust can get a little much for me sometimes as well lol

blake79

Yeah I'm working on getting Borla XR1 long tube headers with the Borla ATAK exhaust. I really want 3.55 though, just hoping to get decent mileage on the highway without having to go like 65 you know?

Steelhounds

I just dI'd 391s in my camaro. Now for me my loud exhaust made it a little over the top. 373 would be something to consider. Maybe start a thread asking members that have 355 or 373 what their opinion is. This eliminates the guessing and uncertainty that's going on here.

alsGT

3.73/3.15 X 2,000 RPM = 2,368 RPM

At 78 MPH you will be at 2,368 RPM.

I went from 3.31 to 4.11 and saw no difference in mileage, street or highway.

blake79

Where did you get the math for that?

Norm Peterson

6th grade math class?

You claimed 2000 rpm @ 78 mph with 3.15 gears. RPM will go up in the same ratio as whatever the new numerically higher axle gear you're thinking of getting is to the one that's in there now.


Norm

Dino Dino Bambino

With stock gearing & tires, 75mph in 6th gear will be at 2050rpm. I think the best balanced gears for your car would be 3.55's and these would raise the rpm to 2300rpm at 75mph.