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Discussion Starter #1
I supposedly had a 3.5 rear end installed by a local shop that replaced the 2.79 in my 1967 8" rear end. However, after adjusting the driven speedometer from a 15 tooth to a 21 tooth to correct the speedometer, the new rear end seems to be closer to a 3.73 than a 3.5.

Is there an easy way to tell what the ratio of the rear end is by counting the number of drive shaft rotations per wheel rotations?

Thanks
 

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If it's a posi you should be able to put a mark on the drive shaft and a corresponding mark on the tailsghaft. Then put a mark on one wheel and a tape strip on the floor below it. With the rear wheels off the ground rotate the drive shaft until the wheel has made one revolution. If you have 3.50 gears the driveshaft should have gone 3.5 times around for one revolution of the wheel.

Like I said in my post, a 20 tooth gear was correct for my 3.50 rear to be accurate in terms of speedometer readout.
 

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Gear Teeth

If you had 15 teeth to start with and the speedo was correct, the proper change would be calculated by

(3.50/2.79)*15

This equates to 18.81. Use a 19 tooth and all will seem normal. A 21 tooth would work with a 3.90.

Understand -- this is relative to what you had. I use a 21 tooth with 3.50 gears and 28.5" tall tires. 24, 25" factory 14" tires would need much less.

My car was originally a 19 tooth gear with 25.2" tall tires.
 
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