Why tire size changes your speedometer
Most vehicles work out speed by counting how fast the wheels, axle or transmission output shaft turn, then multiplying by the distance one tire revolution covers. That distance is set at the factory for the original tire size. Fit a taller tire and each revolution covers more ground, but the computer still assumes the old distance, so the speedometer reads low: you are going faster than it shows. A shorter tire does the opposite. The odometer is affected in exactly the same proportion.
The formula
Because the error is a fixed percentage, it grows with speed: a 3.5% error is about 1 mph at 30 mph but more than 2.5 mph at 75 mph.
Worked example
A truck that came with 265/70R17 tires (31.61 in tall) now has 285/70R17s (32.71 in). The ratio is 32.71 ÷ 31.61 = 1.0349.
When the speedometer shows 60 mph, the actual speed is 60 × 1.0349 ≈ 62.1 mph. The speedometer reads about 3.4% low. After 1,000 miles on the odometer, the truck has really covered about 1,035 miles.
Using a GPS reading instead
If you already have new tires, you can measure the error directly. On a straight, level road, hold a steady speed, such as 60 mph on the speedometer, and note the speed shown by a phone GPS app or satellite navigation unit. Enter both numbers. GPS speed is generally accurate when the signal is good and the speed is steady, but avoid readings taken while accelerating, under bridges or among tall buildings. Take a few readings and average them.
Keep in mind that speedometers are often designed to read a little high from the factory, so a vehicle's error with original tires may not be zero. The GPS method captures the total error, whatever its cause.
What to do about it
- Small errors (around 2% or less): many drivers simply allow for it. Remember that an error that makes you faster than shown can lead to speeding tickets.
- Recalibration: on many modern vehicles, a dealer or a handheld programmer can update the tire size or revolutions-per-mile setting. Older vehicles may need a different speedometer drive gear.
- Odometer and maintenance: uncorrected odometers under-count miles when tires are taller, which affects service intervals, warranty mileage and fuel economy calculations.
- Driver aids: anti-lock brakes, stability control and transmission shift points also rely on wheel-speed information. Large size changes are another reason to recalibrate.
Tire wear also matters. A new, full-tread tire is slightly taller than a worn one, so expect the error to change a little over the life of the tire. To compare tire sizes in full, use the tire size comparison calculator, and see how gearing changes with the gear ratio calculator.
Frequently asked questions
Do bigger tires make you go faster than the speedometer says?
Yes. A taller tire covers more distance per turn, so your real speed is higher than the speedometer shows, by the same percentage as the diameter increase.
How much speedometer error is legal?
Rules on speedometer accuracy vary by country and apply mainly to manufacturers. Drivers are responsible for obeying the speed limit regardless of speedometer error, so it is wise to correct significant errors.
How do I fix my speedometer after a tire change?
Many modern vehicles can be recalibrated with a dealer scan tool or an aftermarket programmer by entering the new tire size or revolutions per mile. Older vehicles may need a new speedometer drive gear.
Does speedometer error affect my odometer?
Yes, by the same percentage. Taller tires mean the odometer records fewer miles than you actually drive.
Is GPS speed accurate enough to check my speedometer?
Generally yes, at a steady speed on a straight road with good satellite reception. Average several readings for the best result.
Last reviewed November 2026. Spotted an error? Tell us.