How to use it
- Type the original size, the one the vehicle was calibrated for. It is on the tire placard in the driver's door jamb.
- Type the new size, metric or in inches.
- Read the table: each row pairs what the speedometer shows with your true speed. Switch to km/h if you prefer.
- Check the odometer line to see how many miles you will under- or over-count every 10,000 miles.
Why a bigger tire makes the speedometer read low
Most vehicles measure speed by counting how fast the wheels or the transmission output shaft turn, then multiplying by the circumference of the original tire. A taller tire travels farther with every turn, but the computer still uses the old number. So the vehicle moves faster than the needle shows, and the odometer logs fewer miles than you really drove. A smaller tire does the opposite.
The math is one line: true speed = indicated speed × new diameter ÷ original diameter.
| Speedometer shows | True speed |
|---|---|
| 30 mph | 31.2 mph |
| 45 mph | 46.8 mph |
| 60 mph | 62.4 mph |
| 70 mph | 72.8 mph |
| 80 mph | 83.2 mph |
After 10,000 miles on the odometer, that truck has really covered about 10,394 miles.
What else changes
- Odometer and trip meter under-count by the same percentage, which shifts maintenance intervals, warranty mileage and the mileage a future buyer sees.
- Fuel economy from the trip computer, and from your own math, comes out lower than reality, on top of any real change from heavier tires.
- Gearing: a taller tire acts like a taller final drive, so acceleration and towing feel weaker. Big jumps, such as 31 to 35 in, often call for new axle gears.
- ABS, traction and stability control compare wheel speeds, so all four tires must be the same size. Large changes can also upset systems that rely on vehicle speed.
Recalibrating the speedometer
On most recent vehicles the speedometer is corrected in software by entering the new tire size. A dealer can do it, and for many trucks and Jeeps there are plug-in programmers that do the same through the diagnostic port. Older vehicles with a cable-driven speedometer need a different drive gear. After any recalibration, check the reading against a GPS speed app on a straight, level road.
Speedometers are rarely exact even on the original tires. Vehicles built to UN Regulation 39, used in Europe and many other markets, must never show less than the true speed, so many read slightly high from the factory. A slightly taller tire can bring the reading closer to reality, so compare with GPS before drawing conclusions.
Questions
How much does a 33-inch tire change my speedometer?
It depends on the original size. From 245/75R17 (31.47 in) to 33x12.50R17, the tire is 4.87% taller: at an indicated 60 mph you are doing about 62.9 mph.
Will bigger tires make my odometer show more or fewer miles?
Fewer. A taller tire covers more distance per turn, so the odometer records less than you really drove, by the same percentage as the diameter change.
Is GPS speed more accurate than the speedometer?
On a straight, level road at steady speed, a phone's GPS speed is usually very close to true speed and a good way to check a recalibration. It lags when you speed up or slow down.
Can bigger tires get me a speeding ticket?
Radar and laser measure your true speed, not what the dashboard shows. With taller tires you are going faster than the needle says, so recalibrate or adjust for the difference.