Tire Size Calculator

What this calculator does

The Tire Size Calculator translates a metric tire code such as 265/70R17 into physical dimensions. It reports the sidewall height, overall diameter, circumference, and the number of revolutions per mile and per kilometer. A comparison mode shows how two tire sizes differ and how the difference affects the speedometer.

The first number in a metric tire code is the nominal section width in millimeters. The number after the slash is the aspect ratio, which is the sidewall height as a percentage of the width. The number after the construction letter is the wheel diameter in inches.

How to use this calculator

1. Choose tire dimensions or compare two tires.
2. Enter the tire width in millimeters.
3. Enter the aspect ratio as a percentage.
4. Enter the wheel diameter in inches.
5. In comparison mode, enter the same three values for the second tire.
6. Click Calculate.

The calculator works with nominal sizes. Actual tires from different manufacturers can vary slightly even when the sidewall code is the same.

Formula

Sidewall height (mm) = width (mm) x aspect ratio / 100
Sidewall height (in) = sidewall height (mm) / 25.4
Tire diameter (in) = wheel diameter (in) + 2 x sidewall height (in)
Circumference = pi x diameter
Revolutions per mile = 63,360 / circumference (in)
Speedometer error = (Tire 2 diameter / Tire 1 diameter - 1) x 100

The comparison table multiplies a range of Tire 1 speedometer readings by the diameter ratio. If Tire 2 is smaller, the speedometer reads higher than the actual speed. If Tire 2 is larger, it reads lower.

Default example

A 265/70R17 tire has a sidewall height of 185.5 mm, or about 7.30 inches. Its overall diameter is about 31.61 inches (802.8 mm), its circumference is about 99.29 inches (2,522 mm), and it turns about 638.1 times per mile.

Comparing 265/70R17 with 265/40R23 gives a Tire 2 diameter of about 31.35 inches. Tire 2 is about 0.82 percent smaller, so a speedometer calibrated for Tire 1 that reads 20 would correspond to an actual speed of about 19.8.

Assumptions and limitations

• The calculation uses nominal tire dimensions, not measured dimensions on a specific tire.
• Tread wear, inflation pressure, load, and manufacturing tolerances change the rolling circumference.
• Wheel width and offset are not considered. A size that fits dimensionally may still rub or handle poorly.
• The speedometer error assumes the speedometer was calibrated for Tire 1 and that the vehicle's electronics are unchanged.
• Many shops recommend keeping the new tire diameter within about 3 percent of the original. This calculator reports the difference but does not decide whether a fitment is safe.

Frequently asked questions

What do the numbers on a tire mean?

In 265/70R17, 265 is the nominal width in millimeters, 70 is the aspect ratio, R means radial construction, and 17 is the wheel diameter in inches. The sidewall height is 70 percent of 265 mm, or 185.5 mm.

Does a larger wheel always mean a larger tire?

No. A larger wheel is often paired with a lower aspect ratio to keep the overall diameter close to the original. The calculator shows the final diameter, which is what affects gearing, clearance, and the speedometer.

How accurate is the speedometer error estimate?

It is a geometric estimate based on the difference in tire diameter. Real speedometers often read slightly high from the factory, and tire wear changes the rolling radius. Use the estimate as a guide, not as a calibration service.

Reference table

Tire size examples

Diameter = wheel diameter + 2 × sidewall height. Sidewall height = width × aspect ratio ÷ 100.

Source

Reference: Calculator.net - Tire Size Calculator. Tire dimension formulas follow the standard metric tire code definitions.