Thread: Rear ratio with Ford AOD Tranny

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TREMEC has a slick formula for rpm:
rpm=(mphxgear ratiox336)/tire diameter in inches

Your AOD has a final 0.67 ratio. If we assume a 30" tire diameter, want 2000rpm at 70mph cruise and solve for the Gear Ratio,2000 = 70mph cruise x 0.67GRx336)/30 you would need a 3.8 ratio. A 3.73 is close, so lets run that, and you get 1959rpm at a 70mph cruise - about perfect, IMO. You need to plug in your actual tire diameter - Tire Rack does not list a 275/75Rx15 as being an available size.

My mistake, at my age I should not try to recall from memory things like tire size.
The tire size I plan on running is a 265/70R15 with an OD of 29.6". I am looking at two 8" center sections that are available locally, one is a 3:40 and the other a 4:11. What are your thoughts on these two ratios?
Thanks

Originally Posted by 32steel

My mistake, at my age I should not try to recall from memory things like tire size.
The tire size I plan on running is a 265/70R15 with an OD of 29.6". I am looking at two 8" center sections that are available locally, one is a 3:40 and the other a 4:11. What are your thoughts on these two ratios?
Thanks

Definitely the 4.11's - 2090rpm at 70mph, compared to 1729 for the 3.40's which will be lugging. You will get much more zip through the gears, but still cruise below 2100rpm for decent mileage. I think you will be unhappy with the 3.40's, especially with a stock engine.