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Old 10-22-2020, 01:48 PM   #10
Benson
Senior Member
 
Join Date: May 2010
Posts: 2,579
Default Re: Differential Gear Ratio Determination

Quote:
Originally Posted by terryatkins View Post
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



fyi:


3.54 = 11 39


4.56 = 9 41
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