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Old 10-22-2020, 09:23 AM   #1
Bob Bidonde
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Default Differential Gear Ratio Determination

Without taking anything apart, and without peeking at the speedometer drive gear case, what is your process to physically determine the differential gear ratio?
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Old 10-22-2020, 09:55 AM   #2
DBSHELTON
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Default Re: Differential Gear Ratio Determination

Mark front pulley, mark tire, roll tire one revolution and count the engine revolutions.
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Old 10-22-2020, 10:32 AM   #3
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Default Re: Differential Gear Ratio Determination

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Originally Posted by DBSHELTON View Post
Mark front pulley, mark tire, roll tire one revolution and count the engine revolutions.
To add; with transmission in 3rd gear.
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Old 10-22-2020, 11:10 AM   #4
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Default Re: Differential Gear Ratio Determination

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Originally Posted by Y-Blockhead View Post
To add; with transmission in 3rd gear.
Yep, I left that out.
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Old 10-22-2020, 11:46 AM   #5
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Default Re: Differential Gear Ratio Determination

If you don't want to roll the car forward (don't have space, etc.) you can jack up one wheel in the back, mark the tire, and count how many revolutions of the crankshaft it takes to make that one tire go around fully 2 times (not 1) with the transmission in High gear. Use the hand crank and have someone watch the tire while you crank (it is easier with all of the spark plugs out).
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Old 10-22-2020, 11:51 AM   #6
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Quote:
Originally Posted by Jim/GA View Post
If you don't want to roll the car forward (don't have space, etc.) you can jack up one wheel in the back, mark the tire, and count how many revolutions of the crankshaft it takes to make that one tire go around fully 2 times (not 1) with the transmission in High gear. Use the hand crank and have someone watch the tire while you crank (it is easier with all of the spark plugs out).
That is what I did on my AA
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Old 10-22-2020, 12:32 PM   #7
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Default Re: Differential Gear Ratio Determination

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
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Old 10-22-2020, 12:59 PM   #8
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Default Re: Differential Gear Ratio Determination

I like that one Terry!
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Old 10-22-2020, 01:42 PM   #9
Kurt in NJ
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Default Re: Differential Gear Ratio Determination

I mark crankshaft pulley, on level ground, put in 3rd with valve stem up on tire, roll car forward till valve stem up again while counting revolutions, --- 3-3/4 about= 3:78
4 plus a little = 4:11, 3-1/2 =3:54 , usually I grab front wheel to move car so I can watch pulley, and feel position of valve stem
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Old 10-22-2020, 01:48 PM   #10
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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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Old 10-22-2020, 02:17 PM   #11
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Default Re: Differential Gear Ratio Determination

Quote:
Originally Posted by DBSHELTON View Post
I like that one Terry!
Just passing it on.
Found it somewhere on on one of the forums.
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