How Tyre Size Changes Your Speedometer Reading
Your speedometer does not measure speed directly. It counts how fast the wheels turn and converts that into a speed using the size of the original tyres. If you fit tyres with a different overall diameter, the car travels a different distance with every turn of the wheel, and the speedometer becomes wrong.
Short version: larger overall tyre diameter makes your speedometer read lower than your real speed. Smaller diameter makes it read higher. Try to stay within about 3 percent of the original diameter.
Why it happens
A taller tyre has a bigger circumference. For each rotation, the car moves further. The speedometer still thinks you are moving the old distance per rotation, so it shows a lower speed than you are really doing. The odometer is affected in the same way, so it will record fewer kilometres than you actually drive.
How to work out the overall diameter
For a size such as 265/65 R17, use this method:
- Sidewall height = width x aspect ratio. Here, 265 mm x 0.65 = 172 mm.
- Rim diameter in millimetres = inches x 25.4. Here, 17 x 25.4 = 432 mm.
- Overall diameter = rim diameter + (2 x sidewall height). Here, 432 + 344 = about 776 mm.
Do the same calculation for the new size and compare the two figures. See how to read the tyre sidewall if the numbers are unfamiliar.
Worked example
| Size | Overall diameter | Difference from 265/65 R17 |
|---|---|---|
| 265/65 R17 | about 776 mm | Original |
| 265/70 R17 | about 803 mm | About 3.4 percent larger |
| 33x12.50 R20 (a 33-inch tyre) | about 838 mm | About 8 percent larger |
With the 3.4 percent larger tyre, a speedometer reading of 100 km/h means a real speed of about 103 km/h. With the 33-inch tyre, a reading of 100 km/h could mean about 108 km/h.
Other effects of changing size
- Braking and stability systems: ABS and traction control use wheel speed, and they may not work as intended with a large difference.
- Gearing: larger tyres make the car feel slower to accelerate and can raise fuel use.
- Clearance: bigger tyres can rub against the wheel arches or suspension.
- Insurance and legal checks: a speed reading that is much lower than your real speed can lead to speeding fines.
See how to choose tyres for an SUV or pickup for what to check before changing size.
How to check your real speed
- Use a GPS speed app on a phone or a car GPS unit.
- Drive at a steady speed on a clear, safe road.
- Compare the GPS reading with the speedometer.
- Repeat at two or three speeds.
Fixing the error
- Stay close to the original size where you can, within about 3 percent of the overall diameter.
- Ask a workshop whether the speedometer can be recalibrated for your vehicle, for example with a programming tool or a correction module.
- Allow for the difference in everyday driving, and remember that your odometer readings and service intervals will be affected too.
Check the load rating too
Whatever size you pick, the tyre must meet or exceed the vehicle's required load index and speed rating. A larger size is not a safe substitute if the load capacity is lower.