Tire Size Calculator
What a tyre size like 225/45R17 means, and how a new size changes your speedometer.
Good to know. A tyre 3% bigger makes the speedometer read 3% low. At 100 km/h, that is really 103.
How it works
In plain words
The sidewall height is the width times the profile percentage. The overall diameter is the rim diameter, converted to millimetres, plus two sidewalls. A larger diameter covers more ground per turn, so the speedometer reads low by the same percentage.
The formula
sidewall = width × profile ÷ 100 diameter = rim × 25.4 + 2 × sidewall true speed = shown speed × new diameter ÷ old diameter
Worked example
205/55R16 is 406.4 + 2 × 112.75 = 632 mm across; 225/45R17 is 431.8 + 2 × 101.25 = 634 mm. The new tyre is 0.38% larger, well within 3%.
Sources
No outside data or rates: this is plain arithmetic.
4 automated tests check its answers on every change.
About the tire size calculator
A tyre size like 225/45R17 is three measurements. 225 is the tread width in millimetres. 45 is the profile: the sidewall is 45% as tall as the tyre is wide, here 101 mm. R means radial construction, and 17 is the wheel rim diameter in inches. From these, the calculator works out the overall diameter, the sidewall height and the circumference.
Compare two sizes to see what a change does. A different overall diameter changes how far the car travels on each turn of the wheel, so the speedometer and odometer are off by the same percentage: with tyres 3% larger, an indicated 100 km/h is really 103. Most tyre fitters recommend staying within about 3% of the original diameter, and many cars’ ABS and stability systems expect it.
“Plus sizing”, fitting a larger rim with a lower-profile tyre, keeps the diameter nearly the same: 205/55R16 and 225/45R17 differ by under half a percent. Lower profiles look sportier and sharpen steering, but ride harder and are more easily damaged by potholes.
Always check the sizes approved for your car, in the handbook or on the sticker inside the driver’s door, along with the load and speed ratings. Fitting sizes not approved for the car can affect insurance.
Questions people ask
What does 225/45R17 mean?
225 mm wide, a sidewall 45% of the width (101 mm), radial construction, for a 17 inch rim. About 634 mm across.
How much can tire size differ?
Most fitters keep the overall diameter within 3% of the original, and the car’s approved sizes are the safe choice.
Do bigger tires affect the speedometer?
Yes. A larger diameter makes the speedometer read lower than your real speed, by the same percentage.