Gearing for a 3-speed versus 4-speed automatic

[B.E. Coyote]

I see that the recommendations for re=gearing the axles of a 3 sp are usually a higher gearing vs the 4 speed.

Is this because the 3 speed has a lower starting point or because the 4 speed has a higher top end.?

 

[TheBoogieman]

Steep OD vs no OD.

 

[B.E. Coyote]

Ah, makes sense. Thanks guys/

 

[Jerry Bransford]

Steep OD vs no OD.

X2. Put another way, the 4-speed's Overdrive has an overly aggressive .69 Overdrive ratio which drops the RPMs down WAY too much which makes the engine struggle. You have to be more aggressive with the axle gearing for the 4-speed to counteract that problem. The .69 OD ratio means the engine rpms are only 69% of what they are when you have a non-OD automatic.