Tire Size Calculator
Enter any tire size to see its real diameter, sidewall height and revolutions per mile. Compare two sizes to check fitment and speedometer error before you buy.
Calculate your tire size
Speedometer check
| Speedometer shows | Actual speed | Difference |
|---|
How to use the tire size calculator
- Enter your current size. Type it (like 225/45R17) or pick width, aspect ratio and rim size from the menus.
- Add the size you are thinking about. Leave “Compare” on to see both tires side by side.
- Read the verdict. Most mechanics keep the new overall diameter within 3% of the original so ABS, traction control and the speedometer stay accurate.
- Check the speedometer table. It shows your true speed when the dash reads 30, 45, 60 or 75 mph.
Not sure what the numbers on your tire mean? Read our guide on how to read tire size, or see whether bigger tires will fit your car.
The formula behind it
Overall diameter (mm) = Rim × 25.4 + 2 × Sidewall
Circumference = π × Diameter
Revolutions per mile = 1,609,344 ÷ Circumference (mm)
Actual speed = Speedometer reading × (New diameter ÷ Old diameter)
Frequently asked questions
How much bigger can I go without problems?
A change within ±3% of the original overall diameter is the common rule of thumb. Beyond that, the speedometer, odometer, ABS and transmission shift points can be noticeably off, and the tire may rub the fender or suspension.
Will a bigger tire change my speedometer?
Yes. A taller tire covers more ground per turn, so you travel faster than the speedometer shows. A shorter tire does the opposite. The table above shows the exact difference for your sizes.
Can I put a wider tire on the same rim?
Usually one size up (about 10 mm) is fine, as long as it stays inside the rim width range the tire maker lists. Very wide tires on narrow rims bulge and handle poorly.
Is this calculator accurate for every brand?
It uses the standard sizing math. Real tires can differ by a few millimeters between brands and tread designs, so check the maker’s spec sheet for exact figures.
Related tools
Results are estimates based on standard tire sizing math and are for general information only. Always confirm fitment with your vehicle manual, tire maker or a qualified tire shop before buying.