TunerTire size calculator
Compare your current tire with a new size. See how the diameter, sidewall and speedometer change.
When your speedometer reads 100 km/h, you're actually going
100 km/h
| Current | New | Difference |
|---|
How it works
Sidewall height is the width times the ratio. The overall diameter is the wheel diameter (in inches × 25.4) plus two sidewalls. The circumference is the diameter × π, and the speedometer error is the ratio between the two diameters.
These are nominal sizes. A real tire can measure a few millimeters different depending on the brand, the wheel width and the pressure.
Plus-sizing: bigger wheels, same overall size
When you fit bigger wheels, you keep the overall diameter close to stock by choosing a lower-profile tire. Here is a classic ladder, compared with a 205/55R16:
| Size | Wheel | Overall diameter | vs 205/55R16 |
|---|---|---|---|
| 205/55R16 | 16" | 632 mm | — |
| 225/45R17 | 17" | 634 mm | +0.4% |
| 225/40R18 | 18" | 637 mm | +0.8% |
| 225/35R19 | 19" | 640 mm | +1.3% |
| 225/30R20 | 20" | 643 mm | +1.8% |
Questions
How do I read a tire size like 225/45R17?
225 is the tread width in millimeters, 45 is the sidewall height as a percentage of that width, R means radial, and 17 is the wheel diameter in inches.
Why does a bigger tire change my speedometer?
Your speedometer counts wheel turns. A taller tire covers more ground per turn, so you go faster than the speedometer shows. A shorter tire does the opposite.
How much can the diameter change?
Many shops try to stay within about 3% of the stock overall diameter, to keep the speedometer, gearing and clearance close to how the car was built. Check what your car and local rules allow.