rear differential ratio

S

samsonized

Well-known member
just bought a 34 cabriolet and i would like to know what the rear end gear ratio is. is there a way to tell without opening the the rear end and counting the teeth
 
The ring and pinon numbers are supposedly stamped on the bottom of the center section of the rear end. Notice I said supposedly because I can't find any numbers on the rear end in my '39 p/u although I've tried on several different occasions.
 
samsonized said:
just bought a 34 cabriolet and i would like to know what the rear end gear ratio is. is there a way to tell without opening the the rear end and counting the teeth
Click to expand...

You can remove the FILL plug on the rear of the center axle housing on the rear end. Take a dab of white paint and dab it on one tooth of the big ring gear. Rotate that gear one full turn, counting the total number of teeth on the ring. Your '34 will most likely have one of three gear ratios.

3.54 = 11/39

3.78 = 9/34

4.11 = 9/37

The number of teeth on your ring gear should match ONE of the last numbers in each of the above. DD
 
Does it have a starting handle? (crank handle)?

If so you can jack up one rear wheel, put it in top (high) gear, and turn the handle until the raised wheel has turned exactly 2 revs. The number of turns of the crank handle equals the axle ratio.

The possibilities are listed above so it should be easy to work out which one you have.

Most likely is 4.11

This video may help.
https://youtu.be/bkvp3Fq21ZM

Mart.
 
Last edited:
Mart said:
Does it have a starting handle? (crank handle)?

If so you can jack up one rear wheel, put it in top (high) gear, and turn the handle until the raised wheel has turned exactly 2 revs. The number of turns of the crank handle equals the axle ratio.

The possibilities are listed above so it should be easy to work out which one you have.

Most likely is 4.11

This video may help.
https://youtu.be/bkvp3Fq21ZM

Mart.
Click to expand...




With the spark plugs out! :rolleyes:
 
Some of the rear differentials have identifying letters, in stead of the the numbers, stamped which identify the original ratio.
 
Mart said:
Does it have a starting handle? (crank handle)?

If so you can jack up one rear wheel, put it in top (high) gear, and turn the handle until the raised wheel has turned exactly 2 revs. The number of turns of the crank handle equals the axle ratio.

The possibilities are listed above so it should be easy to work out which one you have.

Most likely is 4.11

This video may help.
https://youtu.be/bkvp3Fq21ZM

Mart.
Click to expand...

I think you mean to only let the wheel go around once - not twice. If it is a 4.11, then you should be turning the crank a little over 4 revolutions for one revolution of the rear wheel.
 
Bored&Stroked said:
I think you mean to only let the wheel go around once - not twice. If it is a 4.11, then you should be turning the crank a little over 4 revolutions for one revolution of the rear wheel.
Click to expand...

That's true with both wheels up (and they turn together). With one wheel fixed the spider gears come into play.
 
36coupe said:
What about checking the number on the turtle? Will that tell you the ratio?
Click to expand...

It COULD, or MIGHT, but not for sure. Depending on tire size, year of vehicle and even the difference between 4-cylinder and V8....you see several situations where TWO different numbers on turtle could apply, and we haven't even discussed the possibility that someone has changed SOMETHING over the years. This chart will give you some idea of this. Look under the columns labeled "NO. TEETH" DD


103927
 
Thanks for the chart. That explains quite a bit. I have seen mostly 3.78 rears and #19.
I keep hoping I will find 3.54 rear some day.
 
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