GarageCalc How to read your tire size
Every metric tire size follows the same pattern: width / aspect ratio R wheel diameter.
On a 225/45R17 tire, 225 is the section width in millimeters, 45 means the sidewall
height equals 45 percent of that width, and 17 is the wheel diameter in inches. You can find this
marking on the tire sidewall, inside the driver door jamb, or in the owner's manual.
The formulas this calculator uses
- Sidewall height = width x (aspect ratio / 100)
- Overall diameter = wheel diameter + 2 x sidewall height
- Circumference = diameter x pi
- Revolutions per mile = 63,360 inches / circumference in inches
These are nominal values. Real tires vary slightly between brands and with wear and pressure, usually within 1 to 2 percent of the calculated numbers.
Why tire dimensions matter
Changing overall diameter affects more than looks. A taller tire lowers your effective gearing, makes the speedometer read slow, and can rub on the fender or suspension. A shorter tire does the opposite. If you are switching to a different size, check the diameter change with the tire size comparison calculator and keep it within about 3 percent of the original unless you know what you are doing. Changing wheels too? Check the wheel offset calculator so the new setup does not poke or rub.
Frequently asked questions
How do I read a tire size like 225/45R17?
The first number (225) is the tire width in millimeters. The second number (45) is the aspect ratio: the sidewall height as a percentage of the width. The letter R means radial construction, and the last number (17) is the wheel diameter in inches that the tire fits.
How is overall tire diameter calculated?
Overall diameter = wheel diameter + 2 x sidewall height. The sidewall height is the tire width multiplied by the aspect ratio. For a 225/45R17 tire: sidewall = 225 x 0.45 = 101.3 mm, so diameter = 17 x 25.4 + 2 x 101.3 = 634.3 mm, which is about 25 inches.
What are revolutions per mile and why do they matter?
Revolutions per mile is how many times the tire rotates over one mile of driving. It is used to compare tire sizes: a tire with a larger diameter rotates fewer times, which affects your speedometer reading, odometer accuracy, and effective gearing.
Does the actual tire diameter match the calculated one exactly?
Not exactly. The calculation gives the nominal size. Real-world diameter varies slightly by brand, model, tread depth, inflation pressure and load, typically within about 1 to 2 percent. For fitment decisions this calculation is accurate enough, but always check the manufacturer spec sheet for critical clearances.