Tire Size Calculator
Enter the current and new tire sizes. The calculator shows overall diameter, the difference and the effect on the speedometer.
Compare two tire sizes
Enter the tire width, aspect ratio and wheel diameter for each setup. Tire width is in millimetres; wheel diameter is in inches.
Tire dimensions
Overall diameter is calculated from the nominal tire size. Actual tires can vary, and this does not confirm clearance or load rating.
How to read a tire size
225/45R17 means a 225 mm tire width, a sidewall height that is 45% of the width, and a 17-inch wheel. The sidewall is not 45 mm; it is 45% of 225 mm, or 101.25 mm.
Overall diameter is the wheel diameter multiplied by 25.4, plus two sidewalls: 17 × 25.4 + 2 × 101.25 = 634.3 mm. The calculator uses the sizes you enter, including the new wheel diameter.
What changes when the tire size changes
A taller tire raises the car and changes the speedometer. A shorter tire does the opposite. A wider tire changes the sidewall profile and may change clearance, but width alone does not tell you whether the package fits.
The speedometer error is the percentage difference in overall diameter. A tire that is 1.7% larger will make the speedometer read about 1.7% low at the same actual speed.
What this calculator cannot tell you
It does not know the actual tire dimensions, load rating, wheel width, brake clearance, suspension travel or fender clearance. Use the result to compare sizes, not to approve a purchase.
Frequently asked questions
What does 225/45R17 mean?
225 is the tire width in millimetres, 45 is the sidewall height as a percentage of the width, R means radial construction, and 17 is the wheel diameter in inches.
How do I calculate tire height?
Multiply the wheel diameter by 25.4, then add two sidewalls. Each sidewall is the tire width multiplied by the aspect ratio divided by 100.
Does a larger tire change the speedometer?
Yes. A larger overall diameter makes the speedometer read low; a smaller diameter makes it read high. The calculator shows the percentage difference.
Is a wider tire always better?
No. A wider tire can change sidewall support, clearance and steering feel. Match it to the wheel width and the car, and check clearance before buying.
Is tyre height the same as tire height?
Yes. Tyre is the British spelling of tire. The calculation is the same: wheel diameter plus two sidewalls.
Standards and sources
This calculator uses the tire and wheel dimension conventions from ISO 4000-1 and ISO 3911; fitment guidance also references ETRTO.