Final Drive Gearing, Auto vs Manual
 

Here are some comparisons I've made with commonly available differential gear ratios. I hope it will help some of you. This also helps to explain why I believe automatic's need more torque, and manual's, more horsepower.

Manual transmissions
I=3.626, II=2.188, III=1.541, IV=1.213, V=1.000, VI=0.767
6th gear ratio, Differential ratio, and engine RPM's at 60MPH.
0.767 @ 4.10 = 2575 RPM's @ 60MPH
0.767 @ 4.30 = 2701 RPM's @ 60MPH
0.767 @ 4.55 = 2858 RPM's @ 60MPH
0.767 @ 4.77 = 2996 RPM's @ 60MPH
0.767 @ 4.88 = 3065 RPM's @ 60MPH

Automatic transmissions
I=3.538, II=2.060, III=1.104, IV=1.00, V=0.713, VI=0.582
0.582 @ 4.10 = 1954 RPM's @ 60MPH
0.582 @ 4.30 = 2049 RPM's @ 60MPH
0.582 @ 4.55 = 2168 RPM's @ 60MPH
0.582 @ 4.77 = 2273 RPM's @ 60MPH
0.582 @ 4.88 = 2326 RPM's @ 60MPH
0.582 @ 5.404 = 2575 RPM's @ 60MPH

As you can see, it would take changing to a mythical 5.404 to 1 gear and pinion for an automatic to equal the gearing of the manual transmission. That's why automatic's are rated to get such better mileage than the manual's, despite the torque converter losses. I think a 4.88 to 1 ratio would be ideal for an automatic. I could still do 28MPH in 1st @ 7000RPM's, versus 35MPH in 1st @ 7100RPM's for the manual@4.10. This would give me the holeshot from a dead stop, and still better economy at 60 MPH with a 4.88 automatic's 2326 RPM's versus the 4.10 manual's 2575 RPM's.

Whats a hole shot?

The term is used in motorcycle racing, off-road racing (but also motorsport racing in general) for the rider who is the first one through the first turn.

Again, you need to learn what a torque converter does...

I regard my auto to 4.88s and love it. Better acceleration and I still get over 30 mpg at freeway speeds. Best mod I have made.

im buying 4.88s this winter and swapping them in. thats probably gonna be the last thing till the warranty is up for me lol

Does changing the gearing void warranty?

Probably the rear drive train assembly, but would they really notice?

The only thing you need a FD for is motorsport, and if you're doing anything on track you won't ever need to be comparing an AT 6th gear at 60mph, so once again you've demonstrated extremely well what you know - SFA. Automatics need lower 4th and 5th with the same 6th. Then change the FD exactly the same as you would with MT for corner exists.

So the 4.88 seems best for an auto, but what seems best for the manual. The 4.88 or 4.67. I'm NA e85 and pushing 205whp. This is my next mod but I'm deciding which final gear drive is best. 4.56, 4.67, and 4.88. I usually street race from 0 to 100, which gear would be best for that. I also autox every once in a while but not too important.