Speedometer Error Calculator After a Tire Size Change

Why tire size changes the speedometer

Most modern speedometers work out speed from how fast the wheels or transmission output turn. The vehicle is programmed for the original tire size: it assumes a certain distance per wheel revolution. Fit a taller tire and each revolution covers more ground, so you travel faster than the speedometer thinks. Fit a shorter tire and the reverse happens.

Because distance per revolution is proportional to diameter, the error is simply the ratio of the two diameters:

Actual speed = indicated speed × (new diameter ÷ original diameter)

Worked example

Original size 225/65R17 (28.52 in), new size 235/65R17 (29.03 in). The ratio is 29.03 ÷ 28.52 = 1.018, so the speedometer reads about 1.8% low:

The odometer under-counts by the same amount: after 1,000 indicated miles, you have driven about 1,018.

Which size to enter as "original"

Enter the size the vehicle was built with. NHTSA points owners to the Tire and Loading Information label on the driver's door edge or post and to the owner's manual. If the vehicle has a different size fitted now, the speedometer may already be off.

If a previous owner had the speedometer recalibrated for a new size, use that size as the original instead.

Other systems that use wheel speed

Wheel speed sensors also feed anti-lock brakes and stability control. Continental's plus-sizing bulletin notes that a change in rolling circumference can affect speedometer calibration, ABS and stability systems, and that under most circumstances a change within ±3% does not require adjustments. For bigger changes, it advises contacting the vehicle manufacturer.

Checking your speedometer

A phone GPS app or a dashboard GPS shows ground speed independently of tire size. Drive at a steady indicated speed on a straight, level road and compare. Remember that most speedometers are not perfectly accurate even on the original tires, so measure before and after a size change if you want to separate the two effects.

Speedometer error in km/h

The percentage is the same in any unit. Switch the calculator to km/h to see a table at 40, 50, 60, 80, 100 and 120 km/h.

Related tools

• Tire size comparison shows the full dimension table for two sizes.
• Tire size calculator converts a size code to inches.
• Our plus sizing guide covers load capacity, speed rating and pressure when changing sizes.

Common questions

How much does a 1 inch taller tire change the speedometer?

It depends on the starting size. On a 28.5 inch tire, 1 inch is 3.5%, so an indicated 60 mph is about 62.1 mph. On a 33 inch tire, 1 inch is 3.0%.

Will bigger tires make me get speeding tickets?

They can make you go faster than the speedometer shows. At a 3% increase, an indicated 70 mph is about 72 mph. A GPS speed reading is a good real-world check.

Can the speedometer be recalibrated?

On many vehicles a dealer or a programming tool can update the tire size setting. Whether that is possible depends on the vehicle; ask the manufacturer or dealer.

Does tread wear affect the speedometer?

Slightly. As tread wears, the diameter shrinks a little, so the speedometer reads a touch higher than on new tires of the same size.