How this calculator works
A tire size such as 205/55R16 gives the width in millimetres, the sidewall height as a percentage of the width, and the wheel diameter in inches. Sidewall height is the width times the aspect ratio. Overall diameter adds two sidewalls to the wheel diameter. Circumference is pi times the diameter, and revolutions per kilometre or mile divide the distance by the circumference. When comparing sizes, the diameter difference and percentage show how much taller or shorter the new tire is. A speedometer calibrated for the original tire will read low when the new tire is taller, because the car travels farther per revolution.
Formula
Sidewall height = width x aspect ratio. Overall diameter = wheel diameter + 2 x sidewall. Circumference = pi x diameter. Revolutions per km = 1,000 m / circumference. Actual speed = indicated speed x new diameter / original diameter.
Worked example
A 205/55R16 tire has a 112.75 mm sidewall and a 631.9 mm overall diameter, giving a circumference of 1,985 mm and about 503.7 revolutions per kilometre. Compared with a 215/60R17 at 689.8 mm, the new tire is 57.9 mm taller, 9.16 percent larger, and a 100 km/h speedometer reading becomes about 109.2 km/h actual.
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Frequently asked questions
What do the three numbers in a tire size mean?
205/55R16 means a 205 mm nominal width, a sidewall height of 55 percent of the width, and a 16 inch wheel. The R denotes radial construction.
How much diameter difference is acceptable?
A common guideline is to stay within about 3 percent of the original diameter so the speedometer, gearing and clearance remain close to standard. Larger changes may require recalibration and can affect braking and traction control.
Why is the calculated diameter not exactly the same as the manufacturer figure?
Nominal width and aspect ratio are not exact measurements, and the rolling diameter under load is slightly smaller than the unloaded diameter. Use the manufacturer's revolutions-per-mile specification for the most accurate speedometer work.
Does a wider tire always mean a taller tire?
No. Width and aspect ratio together determine sidewall height. A wider tire with a lower aspect ratio can have the same or a smaller overall diameter.