Thread: Rear ratio with Ford AOD Tranny

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Well, my one Son had a Mustang with an AOD and 3:73 gears. It pulled well and 4th gear was fine on the highway. But 4th gear went out on it as so many AOD's do and it was like driving around in second gear because he only had 3rd as his top gear. But when it was going into 4th it was fine.

A lot depends on what level of rpms you are willing to tolerate. I know of some cars running very deep gears with no overdrive and the owners think they are ok. Your tires are fairly tall which cuts down the ratio somewhat.

Don

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.