Tire Size Calculator
Compare your current tires with a new size and see how diameter, sidewall height and speedometer reading change.
- Current diameter—
- New diameter—
- Real speed at that reading—
- New sidewall height—
- Width change—
- New circumference—
- New revolutions—
- Ride height change—
Saved scenarios
100% private. Everything is calculated in your browser. Nothing you type is sent or stored on a server.
Estimates for education only — not financial, tax or legal advice. Lenders may calculate taxes and fees differently by state or province. Always check your contract.
How to read a tire size
A size like 225/45 R17 means a tire 225 mm wide, with a sidewall that is 45% of the width (101 mm), mounted on a 17-inch rim. The R stands for radial construction.
Worked example
225/45 R17 has a diameter of about 634 mm (24.97 in). 235/45 R18 is about 669 mm (26.33 in) — 5.4% larger. With the bigger tires, when the speedometer shows 60 mph you are really doing about 63.3 mph.
How much difference is OK?
Many tire shops recommend staying within about ±3% of the original diameter. Larger changes affect the speedometer and odometer, can confuse ABS and stability control, and may cause rubbing on the suspension or wheel arches. Always check the sizes approved for your vehicle (door-jamb placard or owner's manual) and the load and speed rating.
Bigger tires also change fuel economy readings — re-check them with the gas mileage calculator.
Common tire sizes
See the exact dimensions and equivalents of popular sizes such as 225/45R17, 205/55R16, 235/65R17 and 265/70R17 in the tire size chart.
Frequently asked questions
What does 225/45 R17 mean?
225 is the width in millimeters, 45 is the sidewall height as a percentage of the width, R means radial and 17 is the rim diameter in inches.
How much bigger can I go on tire size?
A common rule is to stay within about 3% of the original overall diameter, and only use sizes approved for your vehicle.
Will bigger tires change my speedometer?
Yes. Taller tires make the speedometer read lower than your real speed; smaller tires make it read higher.
Can I put wider tires on the same rim?
Often one size up in width fits, but every tire has an approved rim width range. Check the tire maker's specifications.
Does the calculator work in millimeters?
Yes. It shows every result in inches and millimeters.
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