TH400 speedometer gears
 

I know there are online calculators where you can figure speedometer driven gears, but does anyone happen to know the teeth counts on a 3/4 ton with 3.54 and 4.10 gears?

Re: TH400 speedometer gears
 

Tire size is needed. Also, there are 2 size and multi count drive gears that go on the output shaft. Without knowing all the variables, impossible to figure.

Re: TH400 speedometer gears
 

Your axle gear ratio determines how many times your driveshaft rotates for every full tire rotation. For example, a 3.73 gear ratio requires 3.73 driveshaft rotations to turn your tires once around. You should be able to find your gear ratio listed on the axle tag, however, if that has gone missing, you can simply count the number of teeth on each gear and divide the number of ring gear teeth by the number of pinion gear teeth.

[ Ring Gear Teeth / Pinion Gear Teeth ] = [ 41 / 11 ] = [ 3.73 ]

TCI® Auto https://share.google/R8IwduoEKFAxBm8Be

This site has drive, driven, and tire size calculators

Re: TH400 speedometer gears
 

I know my axle ratios - one is a 3.54 Dana and the other is a 4.10 Chevrolet (Eaton, as some call it). I just bought a TH400 transmission local and the tag shows it to be out of a 1972 truck. It has a 41 tooth driven gear. Just wondering if it might fit right in with one of these gear ratios or if I need to swap the gear out

Re: TH400 speedometer gears
 

Where you gonna get the new gear?