How to read a tire size
A tire size is a short recipe for three measurements: how wide the tire is, how tall its sidewall is, and what wheel it fits. Once you can read it, you can work out how tall the tire stands.
Metric sizes: 265/70R17
Most tires are marked in metric. Take 265/70R17, optionally with a letter in front and a load and speed rating after it.
| Part | Example | What it tells you |
|---|---|---|
| Letters before | P, LT, ST, T | The tire’s purpose: passenger (P), light truck (LT), special trailer (ST) or temporary spare (T). Sizes without a letter are called Euro-metric and follow the same geometry. |
| Section width | 265 | How wide the tire is, in millimetres, at its widest point. It is measured on a specific wheel width, so on your wheel it can differ a little. |
| Aspect ratio | 70 | The sidewall height as a percentage of the width. A 70 means the sidewall is 70% as tall as the tire is wide. |
| Construction | R | Radial, which is nearly every modern tire. A ZR instead of R historically marked tires built for speeds above 149 mph (240 km/h). |
| Wheel diameter | 17 | The diameter of the wheel the tire fits, in inches. |
| Load and speed | 115S | A load index (how much weight the tire is rated to carry) and a speed letter (S is 112 mph, T 118, H 130, V 149). They do not change the size, but never go lower than the rating your vehicle came with. |
Turning it into inches
Two short formulas give you the sidewall and the overall height of the tire:
sidewall (mm) = width × aspect ratio ÷ 100 overall diameter = wheel diameter + 2 × sidewall
For 265/70R17 the sidewall is 265 × 0.70 = 185.5 mm, which is 7.30 in. The overall diameter is 17 in + 2 × 7.30 in = 31.6 in. That is the number that sets how tall the tire stands in the wheel arch and what your speedometer believes.
Inch sizes: 35x12.50R17
Many truck and off-road tires are marked in inches instead, often called flotation sizes. The first number is the overall diameter, the second is the section width, and the last is the wheel diameter, all in inches. A 35x12.50R17 is 35 in tall and 12.5 in wide on a 17 in wheel, so its sidewall is (35 − 17) ÷ 2 = 9 in.
The two systems describe the same things, only from different starting points. A 12.50 in section is about 318 mm, which is why 315/70R17 is the usual metric stand-in for a 35x12.50R17. It works out to 34.4 in tall, a little under 35. Real tires often measure a little off their nominal size, which is why makers publish measured dimensions.
Common sizes, worked out
| Size | Overall diameter | Sidewall | Revolutions per mile (nominal) |
|---|---|---|---|
| 205/55R16 | 24.9 in (632 mm) | 4.4 in | 811 |
| 225/45R17 | 25.0 in (634 mm) | 4.0 in | 808 |
| 245/40R18 | 25.7 in (653 mm) | 3.9 in | 784 |
| 235/65R17 | 29.0 in (737 mm) | 6.0 in | 695 |
| 265/70R17 | 31.6 in (803 mm) | 7.3 in | 638 |
| 285/70R17 | 32.7 in (831 mm) | 7.9 in | 617 |
| 33x12.50R17 | 33.0 in (838 mm) | 8.0 in | 611 |
| 35x12.50R17 | 35.0 in (889 mm) | 9.0 in | 576 |
These are calculated from the size markings. Tire makers publish measured figures for each model, and they can differ from the calculated ones by a few tenths of an inch. Makers also publish revolutions per mile measured on a loaded, rolling tire, which are usually a few percent higher than the nominal figures here because a loaded tire rolls on a slightly smaller radius. For a speedometer correction only the percentage difference between two tires matters, and it is the same on either basis.
Why the overall diameter matters
Changing tire size changes more than looks. A taller tire raises the vehicle slightly, moves the top of the tire closer to the wheel arch, and covers more road with each turn, which throws off the speedometer. Our speedometer guide covers that last part. A wider tire reaches further toward the fender and the suspension, and how far depends on the wheel too, which the offset and backspacing guide explains.
Enter two sizes and see the difference in height, width and speedometer reading.Open the calculator