Tire Size Calculator
Enter the three numbers from your tire sidewall, such as 225/65R17, to see the overall diameter, sidewall height and revolutions per mile. Enter a second size to see how it changes the speedometer.
How to use this tire size calculator
- Find the size on the sidewall of your tire, for example 225/65R17. The first number is the width in millimeters, the second is the aspect ratio and the last is the rim diameter in inches.
- Enter your current size and the new size you are thinking about.
- Enter the speed you want to check on the speedometer.
- Press Calculate to compare diameter, sidewall height, revolutions per mile and the true speed.
- Use Copy result to share the comparison with a tire shop.
How to calculate tire size and diameter
A tire size such as 225/65R17 describes the width, the sidewall as a percentage of width and the rim.
- Sidewall height = width x aspect ratio / 100 (in millimeters)
- Overall diameter = rim diameter (in) + 2 x sidewall height / 25.4
- Revolutions per mile = 63,360 / (pi x overall diameter in inches)
- Difference = new diameter / current diameter – 1
- True speed = speedometer reading x new diameter / current diameter
Worked example
A 225/65R17 tire has a sidewall of 225 x 0.65 = 146.25 mm. The overall diameter is 17 + 2 x 146.25 / 25.4 = 28.52 inches. A 235/65R17 tire has a sidewall of 152.75 mm and a diameter of 29.03 inches, which is 1.79% larger. If the speedometer reads 60 mph, the true speed with the larger tire is 60 x 1.0179 = 61.1 mph.
Assumptions & limits
- The diameter is calculated from the nominal numbers on the sidewall. A real tire can differ by a little because of the tread design, wear and inflation.
- The speedometer error assumes it was accurate with the current tires. Some vehicles are calibrated to read a little high from the factory.
- The calculator does not check whether a tire fits your vehicle. Clearance, load index, speed rating and rim width all matter, so ask a tire shop or check the vehicle placard.
- Changing the overall diameter also changes gearing, ground clearance and odometer readings.
- The aspect ratio is a percentage of the width, which is how tire sizes are written.
Frequently asked questions
How do I calculate the diameter of a tire?
Add the rim diameter in inches to twice the sidewall height in inches. The sidewall is the width times the aspect ratio, divided by 25.4 to convert millimeters to inches.
What do the numbers on a tire mean?
In 225/65R17, 225 is the width in millimeters, 65 is the sidewall height as a percentage of the width, R means radial and 17 is the rim diameter in inches.
How much does a bigger tire change my speedometer?
The speedometer reads low by the same percentage that the diameter grows. A 1.8 percent larger tire makes a 60 mph reading a true 61.1 mph.
How much difference in tire diameter is safe?
Many sources suggest staying within a few percent. Your vehicle maker or a tire shop can tell you the right limit for your vehicle.
What is revolutions per mile?
It is how many times a tire turns to cover one mile. Larger tires turn fewer times.
Can I use it for the same size from a different brand?
Yes, but real diameters vary a little between brands, so the true figures can differ from the calculated ones.
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Last updated: November 2026