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How to Read a Tire Size
Decode the code on your tire's sidewall, such as 275/55R20, into width, sidewall height and rim size. For anyone about to compare or buy tires.
Example: 275/55R20. Also accepts 275/55-20 or 275 55 20.
- 275
- Section width in millimetres: 275 mm, or 10.8 in.
- 55
- Aspect ratio: the sidewall is 55% as tall as the tire is wide, which is 151 mm or 6.0 in.
- R
- Radial construction, used by almost all modern car and light-truck tires.
- 20
- Rim diameter in inches: the wheel this tire fits is 20 in across.
Together that makes an overall diameter of 31.9 in (810 mm).
The parts of a tire size
A size like P275/55R20 105H packs six facts into one line. From left to right:
- P (optional)
- The type of vehicle. P is passenger, LT is light truck, ST is special trailer and T is a temporary spare. No letter usually means a Euro-metric tire.
- 275: section width
- Width in millimetres from one sidewall to the other. Here 275 mm, or 10.8 in.
- 55: aspect ratio
- Sidewall height as a percentage of width. 55% of 275 mm is 151 mm.
- R: construction
- R means radial. Almost every car and truck tire sold today is radial.
- 20: rim diameter
- The wheel diameter in inches. The tire only fits a 20-inch wheel.
- 105H: load index and speed rating
- See the tables below.
The overall diameter follows from these numbers: 31.9 in for 275/55R20. See the tire size converter for the full list, or compare it with another size.
Load index and speed rating
| Speed rating | Max speed (mph) | Max speed (km/h) |
|---|---|---|
| S | 112 | 180 |
| T | 118 | 190 |
| H | 130 | 210 |
| V | 149 | 240 |
| W | 168 | 270 |
| Y | 186 | 300 |
| Load index | Max load per tire (kg) |
|---|---|
| 91 | 615 |
| 100 | 800 |
| 105 | 925 |
Source: speed ratings and load index tables published by Pirelli and the Northern Territory Government, checked 2026-10-27. Always match or exceed your vehicle's required load index and speed rating.
Where to find the size on your car
- On the tire sidewall. The size is moulded into the rubber in large characters.
- On the driver's door jamb. The placard lists the original tire size, recommended pressure and load limits.
- In the owner's manual. It lists the approved sizes for your model.
The placard is the best starting point when you want to change size. Enter the size from the placard as Current tire size in the tire size comparison calculator and try the replacement as the new size.
Tire size FAQ
What does the P in P225/65R17 mean?
P stands for passenger vehicle tire in the US metric system. LT means light truck. Tires with no letter are Euro-metric and are rated slightly differently for load. The width, aspect ratio and rim diameter are read the same way.
What is the load index?
A number after the size, such as 100 in 225/65R17 100H. It maps to the maximum load one tire can carry: index 91 is 615 kg, 100 is 800 kg and 105 is 925 kg.
What is the speed rating?
A letter after the load index. It shows the highest speed the tire is rated for under the standard test: S is 112 mph, T 118 mph, H 130 mph, V 149 mph, W 168 mph and Y 186 mph. Do not read it as a speed to drive at.
How do I find the age of a tire?
Look for the letters DOT on the sidewall followed by a code that ends in four digits. The first two digits are the week and the last two are the year of manufacture, so 2423 means the 24th week of 2023.