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