Speedometer Gear Calculator

Your details

Choose how you want to enter your tire size.
Overall outside diameter of the mounted tire in inches. Measure from ground to top of inflated tire, or look it up in a tire size chart.
in
Your rear (or front) axle gear ratio, e.g. 3.73 or 4.10. Found on the axle tag or in factory documentation.
Number of teeth on the speedometer drive gear, mounted on the output shaft of your transmission. Common values are 6-9 for GM transmissions.
teeth
If you enter your existing driven gear, we calculate the speedometer error it is causing. Leave as-is if you have not yet measured it.
teeth
Recommended driven gearAcceptable accuracy
18teeth

Nearest whole tooth count - the gear you should install

Calculated tire diameter30in
Tire revolutions per mile672.3
Driven gear (exact)17.54teeth
Speedometer error (recommended gear)2.65%
Error with current driven gear1.34%
Speedo reads at 60 mph (recommended gear)61.6mph
2.65% %
Reads low (unsafe)<-5%Slightly low-5%--2%Excellent-2%-2%Slightly high2%-5%Reads high5%+

Install a 18-tooth driven gear for 2.65% speedometer error.

  • The ideal driven gear is 17.54 teeth. Rounding to 18 teeth gives you 2.65% speedometer error.
  • At true 60 mph your speedometer will read 61.6 mph - slightly high (conservative, which is safer).
  • Your current driven gear is causing 133.8% speedometer error. Replacing it with a 18-tooth gear reduces that to 2.65%.

Next stepA 18-tooth gear is acceptable but not perfect. Verify with a GPS at 60 mph after installation to confirm the reading.

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