Gear ratio

3

31 RPU

Senior Member
Silly Question: What is the.best way to determine the gear ratio in a rear end without taking it apart? Do you simply jack up one wheel and rotate a tire and count the number of turns of your wheel ompared to your drive shaft?
If it is a 3.78 the drive shaft should turn 3.78 turns to the wheel's one turn?
 
31 RPU said:
Silly Question: What is the.best way to determine the gear ratio in a rear end without taking it apart? Do you simply jack up one wheel and rotate a tire and count the number of turns of your wheel ompared to your drive shaft?
If it is a 3.78 the drive shaft should turn 3.78 turns to the wheel's one turn?
Click to expand...

Your procedure is correct but your numbers are wrong because the differential splits the crankshaft rotation in half with one wheel up and one down.

Put a piece of masking tape on the tire at the ground on the wheel that is up. With transmission in high gear (3rd), turn the hand crank and count the number of hand crank revolutions it takes (plus estimated fraction) to bring the tape back around to the floor 2 times. The number of turns of the hand crank is your rear axle ratio.

Your common choices are:

3.25 - about 3 and 1/4 turns of the hand crank

3.54 - about 3 and 1/2 turns

3.78 - about 3 and 3/4 turns

4.11 - and number of hand crank turns over 4

HTH.
 
Thanks,
Looks like I have a 26% mitchell with a newly rebuilt rear end with a 4.11 ring and pinion that I wanted to slide under a Tudor Sedan. 4.11:1 is probably too low a gear to use with the Mitchell. From what I gather, a 3.78:1 is probably best. Is a 3.54:1 too high for the larger body car with a stock engine?
 
31 RPU said:
Thanks,
Looks like I have a 26% mitchell with a newly rebuilt rear end with a 4.11 ring and pinion that I wanted to slide under a Tudor Sedan. 4.11:1 is probably too low a gear to use with the Mitchell. From what I gather, a 3.78:1 is probably best. Is a 3.54:1 too high for the larger body car with a stock engine?
Click to expand...

My opinion is YES! A stock engine would struggle with a 3.45 diff and O/D. Hills would be a real challenge for it, even a good head wind would be too much. A high compression head will go some way to overcoming that. They give you more grunt at the lower end of the rev range, which is where you need it with an O/D. I wouldn't go any higher than 3.78 under a Tudor with stock motor and O/D.
 
I do have a police head, maybe I should pull it out and clean it up..
Anybody have a new 3.78:1 ratio ring and pinion they would like to trade for a 4.11:1?
Mine is in a rear end but has never been run.
PM me.
 
I already have a 26% overdrive. Unless Mitchell would trade out gears with me to convert my overdrive to a 36%? Hmmm
 
How would a 4.11:1 differential and a 36% overdrive compare to a 3.78:1 differential and a 26% overdrive?
 
With overdrive in "low" 1st 2nd and 3rd gears will be lower than with a 3.78:1 differential. When you shift overdrive to "high" then everything is about the same as having a 3.78:1. Is this correct? Would I even use 1st low?
I may be thinking too hard but would shifting from 3rd "low" to 3rd "high" be about the same as 2nd low to 3rd low percentage wise?
Just trying to get an idea about what I want with no regrets.
 
As I read your comment you are treating the mitchel unit as an underdrive not an overdrive. First second and third should be the same in low as it is with out the mitchel. Your advantage comes with the high end of the mitchel. This is where the 26% or 36% increase is. Jack.
 
I am also looking at keeping the 4.11 instead of changing to a 3.78. The overdrive as I understand it changed to a 36% will give the same results as a 3.78 with a 26% overdrive.
I am looking at the whole picture. Will I be satisfied with the 4.11 is the real question?
 
31 RPU said:
I am also looking at keeping the 4.11 instead of changing to a 3.78. The overdrive as I understand it changed to a 36% will give the same results as a 3.78 with a 26% overdrive.
I am looking at the whole picture. Will I be satisfied with the 4.11 is the real question?
Click to expand...

You probably will be because the overdrive will still make a big improvement, and get you down to just over a 3:1 ratio.

It shouldn't be hard to find a good used 3.78 ring and pinion, or complete rear end, because so many are still turning nice Model A's into rods.
 
Assuming you have 21 inch wheels here are the speeds you would obtain with different gearing at 1000 RPM
4.11 rear with no OD 20.966 MPH
3.78 rear with no OD 22.797 MPH
4.11 rear with Mitchell 26% OD 28.333 MPH
4.11 rear with Mitchell 36% OD 32.760 MPH
3.78 rear with Mitchell 26% OD 30.776 MPH
3.78 rear with Mitchell 36% OD 35.609 MPH

Hope this helps.
 
Then easiest way was removing the gear oil plug and marking the ring gear with a crayon. Then turn the wheel while counting the teeth.
Leroy 31RPU in Ca.
 
A stock Model A, with the normal 3.78 rear axle ratio, is already geared fairly low. Unless you were hauling a load or had an unusually heavy car, I cannot imagine why one would want the 4.11 ratio, and then couple with the 36% Mitchell. When I drive my Town Sedan around, with 3.78 ratio, it is already low geared....I cannot imagine having a 4.11, without a specific need.....
 
I know I posted this, don't know what happened to it. If the rear ends are apart, just count the teeth and divide. A 9/34 would be 3.78.
Sorry, I guess I misread the post.
 
Last edited:
lnvmyrick said:
Then easiest way was removing the gear oil plug and marking the ring gear with a crayon. Then turn the wheel while counting the teeth.
Leroy 31RPU in Ca.
Click to expand...

Hey Leroy, that might only work if you know you have a post '28 rear end. I say this, because I have a '28 rear end with a 10 tooth pinion instead of the 9 tooth, which renders mine a 370:1
 
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