Decode and compare tire sizes. This free tire size calculator turns a size like 225/45R17 into overall diameter, sidewall and revs per mile, and compares two tires for diameter change and speedometer error.
Enter a tire size like 225/45R17 as width / aspect / wheel. Compare mode shows how a new size changes overall diameter and what your speedometer really means.
Overall diameter = wheel diameter + twice the sidewall height, where sidewall = width x aspect / 100. A larger overall diameter makes the speedometer read low (you’re actually going faster than it shows).
What This Tire Size Calculator Does
A tire size calculator decodes a size like 225/45R17 into real dimensions – overall diameter, sidewall height, circumference and revolutions per mile. Compare mode also shows how switching to a new size changes overall diameter and affects speedometer accuracy.
How Tire Dimensions Are Calculated
sidewall (mm) = width x aspect / 100diameter (mm) = wheel (in) x 25.4 + 2 x sidewallrevs per mile = 1,609,344 / (pi x diameter in mm)
For example, a 225/45R17 tire has a sidewall of 101.25 mm, an overall diameter of about 24.97 in, and turns roughly 808 times per mile.
Understanding the Size Code
| Part of 225/45R17 | Meaning |
|---|---|
| 225 | Tire width in millimeters |
| 45 | Aspect ratio – sidewall height as % of width |
| R | Radial construction |
| 17 | Wheel diameter in inches |
Why Changing Tire Size Affects Your Speedometer
A speedometer is calibrated to the overall diameter of the tires a vehicle shipped with. Fitting a 245/40R17 in place of a 225/45R17 reduces overall diameter by about 1.02% (roughly 0.26 in smaller) – a smaller tire spins faster for the same true speed, so the speedometer reads slightly high. At a displayed 60 mph, true speed would be about 59.4 mph.
Fitting a larger-diameter tire has the opposite effect: the speedometer reads low, meaning actual speed is higher than what’s displayed.
Why Diameter Matching Matters When Changing Sizes
Keeping the new tire’s overall diameter close to the original – generally within about 3% – helps preserve speedometer accuracy, odometer accuracy, and correct operation of systems like ABS and traction control that rely on wheel speed sensors.
How to Use the Tire Size Calculator
- Enter your tire size as width, aspect ratio and wheel diameter.
- Switch to Compare to check a new size against your current one.
- Add your speedometer reading to see your true speed with the new size.
Planning a road trip on your new tires? See our Road Trip Cost Calculator.
Checks to Make Before Fitting a New Size from the Tire Size Calculator
A tire size calculator tells you whether two sizes have a similar overall diameter, but diameter is only one of several things that decide whether a tire actually fits.
- Clearance: a wider or taller tire can rub the suspension, fender liners or brake calipers.
- Load index and speed rating: the new tire should meet or exceed the ratings on your vehicle’s door placard or in the owner’s manual.
- Offset: wheel offset shifts where the tire sits in the wheel well, which a tire size calculator cannot check.
Also remember that the tire size calculator uses nominal dimensions. Real tires vary slightly, so the speedometer figure from any tire size calculator is an estimate. If you change size substantially, check local rules, tell your insurer and have a tire shop confirm the fit. The tire code article on Wikipedia explains every part of the sidewall marking, which helps before you enter sizes into the tire size calculator.