J Mo

My speedometer is off by 10mph (slow). I realize that I need to replace the plastic gear but how do I know which gear to replace it with? I have a 72 not sure what rear. Is there a way to count teeth in the car's trans now and reduce number of teeth to get the mph right?

MelWff

which transmission?

Knerf

there should be a code stamped on the bottom of your diff that will tell you what gear ratio you have

J Mo

Turbo 400

MelWff

use the calculators on this site to determine the number of teeth you need
http://www.tciauto.com/tc/speedometer-gear-calculator/

You cant go by the code on the rear because someone may have changed the original ring and pinion.
Jack the rear of the car up, place in neutral parking brake off, mark drive shaft with chalk and turn rear wheel one full revolution while counting number of turns of driveshaft. This assume you have a functioning Posi turning both wheels together.

Revi

Wheel and tire diameter will also effect speedometer readings. Original 72 tire diameter is about 27". Is your wheel/tire combination 27"?

With the rear wheels off the ground, car in neutral, you can put a small piece of tape on the tire and a small piece of tape on the drive shaft (this tape is only used to mark a location), spin the wheel one revolution and count how many revolutions the driveshaft makes.

If one wheel revolution equals a tad over 3 driveshaft revolutions, you have a 3.08 rear end.
If one wheel revolution equals a tad under 3.5 driveshaft revolutions, you have a 3.36 rear end.
(You see how this works?)

With a Turbo 400, you most likely have a 3.08 or 3.36 rear end.

http://www.zip-corvette.com/68-82-c3...ven-gears.html

7T1vette

Remove the speedo drive gear from the transmission. Count the teeth on it. If you can purchase one with 2 or 3 less teeth than what you have now, your speedometer should read close to correct. Gear with 3 less teeth would be best; if not available, go with gear having 2 less teeth than what you now have.

Alwyn678

I changed from a TH-400 to a 700-R4. I just do the deduction, but I should fix it.

sunflower 1972

There are 2 speedo sleeves that the speedo driven gear slides into, 35-39 tooth and 40-45 tooth. Make sure you use the correct speedo sleeve if going from a 40 something tooth gear to a 30 something tooth gear.

7T1vette

Hopefully, you can stay in the right range with the internal gear and you can pick a cable drive gear that has 3 fewer teeth (only 2 teeth, if you can't get 3).

J Mo

Thanks 7T1 ! THATS WHAT i WAS THINKING JUST DIDN'T KNOW HOW MANY LESS !