Rear End Gear Ratio
- Thread starter Fozzie
- Start date
Ron in Quincy
Well-known member
Different Ratios used on Model A Fords:
3.70
3.78
3.54
4.11
3.70
3.78
3.54
4.11
Ron Kansas
Member
Is the rear end together or apart?
If apart, simply count the teeth on the ring gear, count the teeth on the pinion gear,
divide the teeth of the pinion gear into the teeth on the ring gear and you got your answer. For example, if you had a 38 tooth Ring gear and a 9 tooth Pinion, you would have a 4.33 ratio. (I am not saying that is a possible combination, just an example)
If the rear end is together and in the car, park the car on a smooth level surface. Put a chalk mark on a rear tire at the exact bottom. Put the transmission in HIGH gear, which is a direct through hook up. Remove the spark plugs -- (Be sure to put the ignition key in your pocket) Use the hand crank to turn the engine allowing the car to move forward. Do not disengage the crank! Count the exact number of turns of the crank until the rear wheel has turned one complete revolution. The number of times the engine turns is the gear ratio.
There are several other ways, but if you remember that the gear ratio is simply the number of revolutions of the pinion gear relative to the revolutions of the ring gear, you can come up with the answer by several means. Also remember that in high gear the engine and the pinion shaft turn the same speed.
Ron, Kansas
If apart, simply count the teeth on the ring gear, count the teeth on the pinion gear,
divide the teeth of the pinion gear into the teeth on the ring gear and you got your answer. For example, if you had a 38 tooth Ring gear and a 9 tooth Pinion, you would have a 4.33 ratio. (I am not saying that is a possible combination, just an example)
If the rear end is together and in the car, park the car on a smooth level surface. Put a chalk mark on a rear tire at the exact bottom. Put the transmission in HIGH gear, which is a direct through hook up. Remove the spark plugs -- (Be sure to put the ignition key in your pocket) Use the hand crank to turn the engine allowing the car to move forward. Do not disengage the crank! Count the exact number of turns of the crank until the rear wheel has turned one complete revolution. The number of times the engine turns is the gear ratio.
There are several other ways, but if you remember that the gear ratio is simply the number of revolutions of the pinion gear relative to the revolutions of the ring gear, you can come up with the answer by several means. Also remember that in high gear the engine and the pinion shaft turn the same speed.
Ron, Kansas
Jim/GA
Well-known member
I do it like Ron, but with one rear wheel jacked up off the ground and the front wheels blocked (so the car does not move). Then I have someone watch the chalk mark on the wheel that is free to turn as I crank the engine. You count the number of turns of the hand crank to get the chalk mark to go around 2 times. That is your rear axle gear ratio.