RE: Differential Gear Ratio Determination
EASILY find Differential Ratio and Compute engine RPM @ a given speed vs the ratio.
Jack up rear end just enough for1 wheel to clear & support with jack stand.
Remove top filler plug, wipe oil off one ring gear tooth and mark it with a dab of white paint. Rotate a tire counting the number of ring gear teeth.
34 = 3.78:1
37 = 4.11:1
39 = 4.56:1
Also see
http://www.fordgarage.com/pages/rearaxleratios.htm
Compute your Engine RPM at a given speed with your axle ratio.
Assumes model A is in third gear (1:1 ratio) and not in overdrive if you have one.
Engine RPM is egual to:
Speed in MPH times Axle Ratio times 336 Divided by the Tire Height. 336 is a constant.
Example: 55MPH, axle ratio 3.78, 30 inch tire height gives Engine RPM of 2,328.48