Tire size calculator
Enter the tire code of the tire you have and of the tire you are looking at to see the diameter of each, how much it changes, and the real speed when the speedometer shows a speed you choose.
This answer is for the example values in the boxes above. 205/55R16 and 225/50R17, speedometer at 60 mph. Change the boxes to see your own answer.
The diameter is the rim plus twice the sidewall, and the turns to a mile are 63,360 inches over the circumference. The real speed is for a speedometer that was set for the tire you have and counts the turns of a wheel.
How it works
The calculator reads each tire code as a width in millimeters, an aspect ratio in percent and a rim in inches. The sidewall is the width times the aspect ratio, and an inch is exactly 25.4 millimeters, so dividing by 25.4 gives its height in inches. The overall diameter is the rim plus twice the sidewall.
The turns to a mile are the 63,360 inches in a mile over the circumference, which is pi times the diameter. The last rows compare the new tire with the one you have.
What the speedometer shows
A speedometer that was set for the tire you have and counts the turns of a wheel reads too low when the new tire is larger, because the wheel goes farther in each turn. The calculator multiplies the speed you type by the new diameter over the old one to give the real speed.
It does not know how the speedometer of your car is set. The owner’s manual of the car says more. A real tire differs a little from its code, and it wears.
A worked example
A 205/55R16 has a sidewall of 112.75 millimeters, which is 4.4 inches. Its diameter is 24.9 inches, or 632 millimeters, and it turns 811 times in a mile. A 225/50R17 has a sidewall of 112.5 millimeters, 4.4 inches too, on a rim that is an inch larger. Its diameter is 25.9 inches and it turns 780 times in a mile.
That is 0.98 inch, or 3.94 percent, larger, and 60 mph on the speedometer is 62.4 mph. A 225/65R17 against a 245/60R18 is 28.5 inches against 29.6, a change of 3.71 percent, and 65 mph on the speedometer is 67.4 mph.
What it leaves out
The calculator uses the sizes in the codes. It does not say whether a tire fits your car or whether its load rating is enough, and it does not look a car up by its make and model.
Sources
- 49 CFR 571.109, Standard No. 109, New pneumatic and certain specialty tires, read on 2026-11-02.
- Wikipedia, Tire code, read on 2026-11-02.
- NIST Special Publication 811, Appendix B.8: factors for units listed alphabetically, read on 2026-11-01.
- NIST Handbook 44, Appendix C, General Tables of Units of Measurement (copy archived on 2009-01-09), read on 2026-11-02.
Questions
What do the numbers on a tire mean?
A code such as 225/45R17 gives the width of the tire in millimeters, the height of the sidewall as a percent of that width, and the diameter of the rim in inches. Here that is 225 millimeters, 45% and 17 inches.
How do I work out the diameter of a tire?
Multiply the width in millimeters by the aspect ratio over 100 to get the sidewall in millimeters, divide by 25.4 for inches, double it and add the rim. A 205/55R16 has a sidewall of 4.4 inches and a diameter of 24.9 inches.
Will my speedometer be wrong with a different tire size?
If it was set for the tire you have and counts the turns of a wheel, yes. A larger tire goes farther in a turn, so the real speed is the shown speed times the new diameter over the old one. A 225/50R17 in place of a 205/55R16 is 3.94% larger, so 60 mph on the speedometer is 62.4 mph.
How many times does a tire turn in a mile?
A mile is 63,360 inches. Divide that by the circumference of the tire, which is pi times its diameter. A 205/55R16 turns 811 times in a mile and a 225/50R17 turns 780 times.
What is the section width of a tire?
It is the distance between the outsides of the sidewalls of an inflated tire, without labels, decoration or protective bands, as the federal rule on tires defines it. The number in the code is the nominal width, and a real tire differs a little.
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