Tire Size Calculator
Type a tire size such as 225/65R17, or tick Enter manually and fill in the width in mm, the aspect ratio and the rim size in inches. The calculator shows the sidewall height, the overall diameter in mm and inches, the circumference and the revolutions per mile. Add a second size to compare the two. You then see the difference in diameter, in mm and in percent, and your real speed when the speedometer, set for the first tire, shows 60 mph.
Tire 1
Tire 2 (comparison — optional)
How to use the Tire Size Calculator
- Enter your numbers in the Tire Size Calculator above.
- Press the Calculate button to see the result.
- To try other numbers, change any value and press the button again.
The tire size math
Sidewall = section width × aspect ratio / 100 · · · Overall diameter = rim diameter + 2 × sidewall · · · Revolutions per mile = 1,609,344 mm / (π × diameter)
A tire marked "P225/65R17" is 225 mm wide, its sidewall is 65% of that width, 146.25 mm, and it fits a 17-inch rim. Overall diameter = 17 × 25.4 + 2 × 146.25 = 724.3 mm, or 28.52 inches. A mile is 1,609,344 mm, so the revolutions per mile are that distance divided by the circumference. The size box takes metric sizes in this pattern, with or without the P.
Worked example
Tire 1 is a 225/65R17 with a diameter of 724.3 mm (28.52"). Tire 2 is a 245/70R17 at 774.8 mm (30.5"). The calculator shows tire 2 as 50.5 mm, or 6.97%, taller. It turns fewer times per mile, so when the speedometer shows 60 mph you are doing 64.2 mph.
Frequently asked questions
Can I go up a tire size?
The calculator shows how much a new size changes the diameter and the speedometer. It cannot tell you whether that size fits your car or is approved for it. Check that with your vehicle maker or a tire dealer before you buy.
Does tire size affect MPG?
The calculator does not estimate fuel economy. It shows the revolutions per mile of each tire. A taller tire turns fewer times per mile, so the speedometer and the odometer read low by the same share.
How do I read tire sidewall?
Take "225/65R17 102H". The tire is 225 mm wide, its sidewall is 65% of the width, R means radial, it fits a 17-inch wheel, 102 is the load index and H the speed rating. Type only the size part, 225/65R17, into the calculator.