Speedometer calculator for your Chevrolet
This will help you pick the right speedometer gears combination for your classic
Vehicle Information
Numbers must be entered as decimals, example 71 1/4" would be entered as 71.25.

Enter your tire size (the diameter may vary slightly depending on the manufacture if in doubt use manufactures posted diameter)

Metric width or Enter Rear Tire Diameter ----> Rear Tire
Profile or Enter Rear Tire Diameter ---->
Rim diameter or Enter Rear Tire Diameter ---->
Effective tire diameter
The tire diameter is less with a load (effective tire diameter = rim diameter + (tire sidewall x 0.94))
Pick your rear end gear ratio
Select your transmission drive gear Your speedometer cable driven gear should have

Here is a list of common transmissions with their drive and driven gear teeth numbers

Transmission Drive Gear Driven Gear
200-4R 10-13 26-31
TH350 7-10 17-25
TH400 15-21 32-45
700-R4 15-20 32-45
4L80E 15-18 34-45
Muncie 6-9 17-25
T-10 6 and 8 20-25 limited sizes available depending on drive gear O.D.
T-5 , T56 and Tremec 7-9 17-22