Speedometer Calibration Calculator
Work out how far off your speedometer reads after a tire or gear change, and what it takes to correct it.
What changed
- Taller tires make the speedometer read low. You are going faster than it says.
- The odometer is wrong by the same percentage, which affects warranty mileage, service intervals and resale.
- Numerically higher gears and taller tires push the error in opposite directions and can cancel out.
Why the reading drifts
A speedometer counts driveshaft or output shaft revolutions and converts them to road speed using a fixed assumption about how far the vehicle moves per revolution. That assumption is built from the original tire diameter and axle ratio.
Change either and the assumption is wrong. Taller tires cover more ground per revolution, so the vehicle is going faster than the instrument believes. Numerically higher gears spin the shaft faster for the same road speed, which pushes the error the other way.
They can cancel out
This is the useful part. If you fit taller tires and re-gear to suit, the two errors work against each other. Go from 28 inch tires on 3.73 gears to 31 inch tires on 4.10 gears and the speedometer ends up within about hundredth of where it started, because both changes were roughly proportional.
That is another argument for re-gearing after a tire change. You get the driveability back and the instruments stay honest.
Fixing it
Modern vehicles are corrected electronically, either with a handheld programmer that rewrites the tire size parameter or with an inline recalibration module. Older cable driven setups use a physical gear in the transmission tailshaft, and you change the driven gear to suit.
Frequently asked questions
How much does a bigger tire affect the speedometer?
Proportionally to the diameter change. Going from 28 inch to 31 inch tires is about 10.7% taller, so at an indicated 60 mph you are actually doing about 66. The odometer under-records by the same 10.7%.
Which way does the error go with bigger tires?
The speedometer reads low, meaning you are travelling faster than it shows. That is the direction that gets people speeding tickets, and it also means your odometer is quietly under-counting the miles you drive.
Does re-gearing fix the speedometer?
It can, if the gear change is proportional to the tire change. Taller tires and numerically higher gears move the error in opposite directions, so a well matched combination lands close to correct. Enter both changes above to see.
Is the odometer wrong too?
Yes, by exactly the same percentage, because it counts the same revolutions. A 10% error means 10,000 recorded miles is really 11,000 driven, which matters for warranty coverage, service intervals and what you tell a buyer.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.