Gear Ratio - Differential

[Bob Grieb]

I have a 1930 Model A pickup and I want to know the gear ratio of the differential. As the drive shaft is enclosed I am not sure how to verfy/determine the gear ratio.

Any help is appreciated

Thanks
Bob

[Dick M]

Place a piece of tape on rear passenger tire and run the tape onto the floor below to create a reference line. Cut the tape between the tire and the floor. Place a reference mark on the crank pully for a starting point. Next, jack up the rear passenger tire until it is just off the floor and place the car into high gear. Turn the engine over with the key off and count how many times the crank pully goes around as the rear passenger tire makes one revolution. If the crank pully goes around 3.27 times you have a 3.27 to 1 ratio; 3.54 times you have a 3.54 to 1 ratio; 3.78 times you have a 3.78 to 1 ratio; and, 4.11 times you have a 4.11 to 1 ratio.

[junk yard kid]

since its only one wheel wont it be double

[Dick M]

You are correct. It would be double. So, jack up both wheels. Sorry, it has been some time since I have done this exercise.

[Bob Grieb]

Originally Posted by Dick M

You are correct. It would be double. So, jack up both wheels. Sorry, it has been some time since I have done this exercise.

Thanks Dick and Junkyard. My priject for morning.

Bob

[Tom Wesenberg]

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Actually you're better off jacking one wheel and counting two revolutions of that tire. With two wheels jacked up one wheel may turn more than the other.

[Bob Grieb]

Originally Posted by Tom Wesenberg

Actually you're better off jacking one wheel and counting two revolutions of that tire. With two wheels jacked up one wheel may turn more than the other.

Tom
Are you saying that if I had a 4:11 rear end ratio the engine would turn 4.11 times to turn the one wheel two revolutions?
Thank Bob

[Tom Wesenberg]

Originally Posted by Bob Grieb

Tom
Are you saying that if I had a 4:11 rear end ratio the engine would turn 4.11 times to turn the one wheel two revolutions?
Thank Bob

Correct.

[socalplanedoc]

I'm guessing it should be in high gear too?

[theHIGHLANDER]

Originally Posted by junk yard kid

since its only one wheel wont it be double

This always drives me nuts. I know it's the way, but I can't get my head around why it takes 2 turns. Considering the function of a differential, one wheel can't be turned by the driveshaft, one can, so now one wheel is a 2.55:1 ratio? Why? You're turning the ring and pinion. Suppose you use the axle side that's better fixed to the differential. Also, sometimes I get the exact ratio by turning 1 wheel 1 time, but I can't recall which axles those were that it worked on (thinking GM 10bolt). Any input?

[Tom Wesenberg]

Originally Posted by theHIGHLANDER

This always drives me nuts. I know it's the way, but I can't get my head around why it takes 2 turns. Considering the function of a differential, one wheel can't be turned by the driveshaft, one can, so now one wheel is a 2.55:1 ratio? Why? You're turning the ring and pinion. Suppose you use the axle side that's better fixed to the differential. Also, sometimes I get the exact ratio by turning 1 wheel 1 time, but I can't recall which axles those were that it worked on (thinking GM 10bolt). Any input?

I'm not sure where the 2.55:1 came from, but when one wheel is held still the gears in the differential make the other rear wheel spin twice as fast, so you need to have the tranny in high gear, then count the times the engine turns over to make ONE rear wheel spin exactly twice around.

No rear axle is more directly connected to the differential than the other axle. Both axles are equally free to spin or lock up, if the rear is up on jack stands. If the brake shoes have some drag on one side only, then that axle will likely stay still while the other spins freely. Or, one axle might turn 1/3 of a turn then hit a tight spot and stop while the axle on the other side starts spinning twice as fast. That's why it's best to just leave one tire on the ground and jack up one side to check the rear end ratio.

[Bob Grieb]

Originally Posted by Tom Wesenberg

I'm not sure where the 2.55:1 came from, but when one wheel is held still the gears in the differential make the other rear wheel spin twice as fast, so you need to have the tranny in high gear, then count the times the engine turns over to make ONE rear wheel spin exactly twice around.

No rear axle is more directly connected to the differential than the other axle. Both axles are equally free to spin or lock up, if the rear is up on jack stands. If the brake shoes have some drag on one side only, then that axle will likely stay still while the other spins freely. Or, one axle might turn 1/3 of a turn then hit a tight spot and stop while the axle on the other side starts spinning twice as fast. That's why it's best to just leave one tire on the ground and jack up one side to check the rear end ratio.

Thanks Tom

[Dave in Petaluma]

How about leaving everything on the ground, pulling the spark plugs, marking the pulley and rear tire, and just push the car one revolution? It's not difficult to move the car by turning a front wheel while watching the pulley go 'round.

[Dodge]

If your speedo was working correctly you can also look at the drive housing it might have the ring and pinon ratio on it. or count the drive teeth the drives have to be
matched to the rear end.