Tire Size Tool

How the math works

Everything on this site comes from a few textbook formulas. Nothing is looked up from a manufacturer database, so real tires can differ slightly from these numbers: measured diameter depends on the brand, tread depth, rim width and inflation.

Reading a metric size

In LT265/70R17, LT means light truck (a P means passenger; no letter is common too), 265 is the section width in millimetres, 70 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the rim diameter in inches. Load and speed ratings after the size (like 121/118S) do not change the dimensions and are ignored.

The formulas

Worked example: 265/70R17

Sidewall = 265 × 70 ÷ 100 = 185.5 mm (7.30 in).
Diameter = 17 × 25.4 + 2 × 185.5 = 431.8 + 371 = 802.8 mm, or 31.61 in.
Revolutions per mile = 63,360 ÷ (π × 31.61) ≈ 638.

Flotation sizes

A flotation size like 33x12.50R17 states the overall diameter (33 in), section width (12.5 in) and rim (17 in) directly. The sidewall is (33 − 17) ÷ 2 = 8 in. The same 33 in tire on a 15 in rim has a 9 in sidewall. To convert to metric, the width is multiplied by 25.4 and the sidewall divided by the width gives the aspect ratio, which is then rounded to the nearest standard metric step.

Speedometer error

A speedometer counts wheel revolutions. A taller tire covers more ground per revolution, so the car is moving faster than the gauge shows; a shorter tire does the opposite. The difference is proportional: a tire 3% taller means you are going 3% faster than indicated, and the odometer under-counts by the same 3%.

Things the math cannot tell you

Whether a size fits your vehicle depends on wheel offset, suspension, load rating and more. Use these numbers as a starting point and confirm fitment with the tire maker or a tire shop.