Tire Size Calculator

Tire size
Tire size
mm
%
in

Enter a tire size like 225/45R17 as width / aspect / wheel. Compare mode shows how a new size changes overall diameter and what your speedometer really means.

Overall diameter
—
Enter a tire size
Diameter
—
Sidewall height
—
Circumference
—
Revs per mile
—

Overall diameter = wheel diameter + twice the sidewall height, where sidewall = width × aspect ÷ 100. A larger overall diameter makes the speedometer read low (you’re actually going faster than it shows).

Use this free tire size calculator to find a tire’s overall diameter, sidewall height and circumference, and to compare two sizes for diameter change and speedometer error.

What this tire size calculator does

A tire size calculator turns a size like 225/45R17 into real dimensions — overall diameter, sidewall height, circumference and revolutions per mile — and can compare a new size against your current one.

How tire dimensions are calculated

sidewall (mm) = width × aspect ÷ 100
overall diameter = wheel(in)×25.4 + 2 × sidewall
speedometer: true speed = indicated × newØ ÷ oldØ

So 225/45R17 has a 101.25 mm sidewall and a 24.97 in overall diameter (about 808 revolutions per mile).

Reading a tire size

Part Example Meaning
Width 225 Section width in mm
Aspect 45 Sidewall is 45% of width
Wheel R17 17-inch wheel diameter

Tire size and your speedometer

Fitting a larger overall diameter makes the speedometer read low — you’re actually going faster than it shows. A smaller diameter does the opposite. Keep within about 3% of the original diameter to stay accurate.

How to use the tire size calculator

  1. Enter width, aspect and wheel for your tire.
  2. Switch to compare to add a second size.
  3. Read the diameter change and true speed.

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