Speedometer Calculator
When you change tire size or regear the axle, your speedometer reads wrong. This speedometer calculator takes the original and new tire diameters (or the old and new axle ratios for a speedometer gear calculator workflow) and returns your true ground speed, the error percentage, and the corrected speedometer reading. Works for trucks with larger off-road tires, race cars with shorter final drives, or any vehicle where someone has asked is my speedometer accurate after a change.
Tire-size speedometer error
A speedometer measures how fast the driveshaft (or output shaft) is turning. The vehicle computer converts that to MPH assuming the stock tire diameter. Put a taller tire on and each revolution covers more ground, but the speedometer still thinks it covered the stock distance. Result: speedometer reads low, you are actually going faster.
The formula is actual speed = indicated speed x (new tire diameter / original tire diameter). For a truck with 28 inch factory tires swapped to 31 inch off-road tires, at an indicated 60 MPH: actual = 60 x (31 / 28) = 66.43 MPH. The speedometer is reading 6.43 MPH (about 10 percent) slow.
Gear-change (regear) speedometer error
If you swap a 3.73 rear axle for a 4.10 to improve low-end torque, the driveshaft spins faster for the same ground speed. A gear-sensing speedometer now reads high. The formula flips: actual speed = indicated speed x (original ratio / new ratio). At an indicated 60 MPH with a 4.10 swap from 3.73: actual = 60 x (3.73 / 4.10) = 54.58 MPH. The speedometer is reading 5.4 MPH high.
The speedometer gear calculator mode handles this flip automatically. For a transmission speedometer calibration (the old driven-gear kind in a manual Ford or Jeep), the same math applies: new driven gear teeth over old driven gear teeth.
Why does my car speedometer not accurate anymore?
Three common reasons. First, you changed tire size (the biggest offender). Second, you regeared the axle. Third, the tire has worn or is underinflated, reducing effective diameter by up to 3 percent. Even a stock vehicle reads 1 to 3 percent high from the factory because regulators in most countries allow a positive bias (speedometer reads slightly high to reduce speeding tickets), but not a negative bias.
Speedometer calibration tool options
Once you know the error percent, you have three ways to fix it. One: a tuner or handheld programmer (SCT, Bully Dog, Hypertech) updates the tire size in the ECU. Two: a dedicated inline speedometer calibration tool like the Yellow Box or TrueSpeed. Three: for older vehicles, swap the transmission speedometer driven gear to a different tooth count. Search for speedometer calibration near me if you need a shop to do it.