identifying rear gear ratio in 1931 model A

T

Tootall

Junior Member
I am looking to purchase a Model A chassis and it has a transmission still attached. Ii wish to know the rear gear ratio and upon looking on the Speedo, I found the numbers 9-34. I am assuming this is an after market or special rear gear ratio.
Could someone help me out?
 
Kurt in NJ said:
3:78
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Yes, the 9-34 you found refers to the number of gear teeth in the rear differential. Different years, or more accurately have different ratios, this done by varying the gears.

My truck originally was 4.11. Now with high speed gears 3.54.

The speedo gears on mine are marked 9-34, but I mentally add a couple of mph. Now the proper speedo gears for the high speed rear are available.

You can confirm your ratio with your rear wheel (one) up on a jack, a starting crank, your transmission in 3rd gear, and understanding that a differential with one wheel held to the ground "doubles" the motion for the free wheel.

Joe K
 
Here’s info from an old post
 

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When you turn the engine over with the hand crank, take the plugs out so it is easier to turn over. To be more accurate you can crank for 20 turns of the rear wheel that is up in the air. The ratio is multiplied by 10. So a 3.78 gear ratio would result in 37.8, or about 37 3/4 turns of the hand crank, etc. It is easier if you have a helper to let you know when the rear wheel has turned 20 times.
 
"Higher mathematics" not being my favorite subject in school, I followed the advice of an old-timer how to determine the Model A rear end gear ratio. Put the rear end on jack stands, put the car in neutral and take out the upper plug from the center carrier (banjo). Wipe the gear teeth visible through the hole with lacquer thinner to make them squeaky clean. Use a light-colored spray paint to cover one of the gear teeth so that is recognizable as the first tooth. Count that as number 1 tooth. With a hefty flat blade screwdriver, step the gear through its 360 degree range, one tooth at a time. It will turn easily. Count the number of teeth until you get back to the painted one. If the number is 34, you have the standard 3.78 ratio. Chances are, that's what your car has. Other ratios are not encountered as often unless a restorer changed ring and pinion gears. The next most commonly encountered ratio among restored "driver" cars would be the 3.54 ratio, which has 39 ring gear teeth.

I like using this ring gear teeth counting method because it is quick, bullet-proof accurate and no need to worry about turning a rear wheel or the engine too far or too short and ending up with a confusing result. Other methods described here will also work, but once the car is up on jackstands, you can do the whole teeth counting procedure lying on your back, or if you're short enough, seated. No jumping around up and down, back and forth, or back-breaking hand-cranking.
Marshall
 
Marshall, The ratio is the summation of Hc divided by 2 times Wt. No higher mathematics needed. So put that in your pipe and smoke it. (In case anyone doesn't get it, I am trying to be funny.)
 
Here in southern N.H. I've never found a 4.11 gearset native to the area.In the 70's I took dozens of rear ends apart,I worked in a junkyard for a while,and saw lots of them go through there.I was mostly after axles,the carnival guys liked them for tent stakes.Never found any 3.54 gears either,even though they became available around the time of the war.I took a set of 4'11's out of a pickup,but the owner had gotten it from his grandfather,who had bought it new in Colorado.We actually tossed the low gears in the scrap pile,they had no value then.
 
To augment Bob's diagram, it should be mentioned that the 3.70 gearset was only used in early 1928 Model A's before switching to 3.78 gears during the year. That's one reason why early 1928 Model A's were reputed to have a higher top end than with later gear sets.
Marshall
 
Marshall V. Daut said:
To augment Bob's diagram, it should be mentioned that the 3.70 gearset was only used in early 1928 Model A's before switching to 3.78 gears during the year. That's one reason why early 1928 Model A's were reputed to have a higher top end than with later gear sets.
Marshall
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Pretty sensitive seat of the pants. That's a whopping 2% difference!
 
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