Performance and units · free calculator
Tyre size calculator and speedometer difference
Enter your current tyre size and, optionally, an alternative size. The calculator shows the dimensions of both and how much your speedometer reading will change.
Result
Diameter difference+0.46%
| Size | 225/45 R17 | 225/40 R18 |
|---|---|---|
| Sidewall height | 101.3 mm | 90.0 mm |
| Overall diameter | 634.3 mm · 24.97 in | 637.2 mm · 25.09 in |
| Circumference | 1,993 mm | 2,002 mm |
| Revolutions per km | 502 | 500 |
- Diameter difference
- +2.90 mm
- Real speed at an indicated 100 km/h
- 100.5 km/hat 60 mph: 60.3 mph
- Within ±3% of the current size
- Yescheck the sizes approved for your car
Example with typical values — change the fields to calculate your own.
How to read a tyre size
In 225/45 R17, 225 is the tyre width in millimetres, 45 is the aspect ratio — the sidewall height as a percentage of the width — R means radial construction and 17 is the rim diameter in inches.
- Sidewall = 225 × 45% = 101.3 mm
- Overall diameter = 17 × 25.4 + 2 × 101.3 = 634.3 mm
- Circumference = π × diameter = 1,992.7 mm, or about 502 revolutions per km
Speedometer difference
The speedometer is calibrated for the original rolling circumference. With a larger tyre the car travels further per wheel revolution, so the real speed is higher than displayed: real speed = displayed speed × new diameter ÷ old diameter. Most tyre fitters recommend keeping the difference in overall diameter within about ±3%; always check the sizes approved for your car (owner's manual, door-pillar sticker or certificate of conformity).
Frequently asked questions
What does 225/45 R17 mean?
225 mm tread width, a sidewall 45% as tall as the width (101 mm), radial construction and a 17-inch rim.
How much can the tyre diameter change?
As a rule of thumb within about ±3% of the original diameter, and only to sizes approved for your car.
Will bigger wheels change my speedometer?
Only if the overall diameter changes. A plus-size such as 225/45 R17 → 225/40 R18 changes the diameter by only about 0.5%, so the speedometer stays almost the same.