Tire size calculator
Compare your tire size with a new one: overall diameter, sidewall, revolutions per mile and the speedometer difference.
The size is printed on the tire sidewall and on the label in the driver door frame.
Comparison
Difference in diameter: –
When your speedometer shows 60 mph, you are really driving –.
| Now | New | Difference | |
|---|---|---|---|
| Overall diameter | – | – | – |
| Overall diameter (mm) | – | – | – |
| Tread width | – | – | – |
| Sidewall height | – | – | – |
| Circumference | – | – | – |
| Revolutions per mile | – | – | – |
Worked out from the nominal size. Real tires of the same size differ a little by brand and tread wear.
Before you change tire size
- Check the sizes your car maker approves (driver door label or owner's manual).
- Make sure the new tire clears the suspension and fenders at full steering lock.
- Keep the same size on each axle, and on all four wheels on most all-wheel-drive cars.
Tire size questions
How do I read a tire size like 225/45R17?
225 is the tire width in millimeters. 45 is the aspect ratio: the sidewall height is 45% of the width. R means radial construction. 17 is the wheel (rim) diameter in inches.
Why does a bigger tire change my speedometer?
The speedometer counts how often the wheels turn. A taller tire travels farther per turn, so you drive faster than the speedometer shows. A smaller tire does the opposite.
How much bigger can I go?
Many tire shops use about 3% difference in overall diameter as a limit. The real limit is what fits your car without rubbing and what the maker allows: check the tire placard in the driver door frame, the owner's manual or a tire dealer.
Does the load index and speed rating matter?
Yes. A new tire should have at least the load index and speed rating the car maker lists for your car. This calculator compares sizes only.