How to read a tire size
A tire size such as LT285/70R17 121/118S has seven parts. This guide tells you what each part means, and it gives the official source for each part.
Tire type: P, LT or no letter
The letter P shows a passenger tire size from the Tire and Rim Association (T&RA).1 A size with no letter at the start can be a size from ETRTO or JATMA.1 Toyo calls these tires hard-metric tires.2
The letters LT show a light truck tire.3 Toyo tells you that LT tires are for light truck or SUV use only.2 Do not compare only the numbers of two sizes. Compare the load index of each tire too.
Section width: the first number
The first number is the nominal section width in millimeters.1 In 285/70R17, the section width is 285 mm, or 11.22 in. The section width is the distance between the outer surfaces of the sidewalls of an inflated tire. It does not include letters, decoration or protective bands.4
A new tire can be a little wider than its nominal width. For passenger tires, FMVSS No. 139 sets a limit of 7 percent over the specified width. For tires in the 240 to 350 kPa pressure classes, the limit is 7 percent or 10 mm (0.4 in), whichever is larger.5
Aspect ratio: the second number
The second number is the aspect ratio. It is the sidewall height as a percentage of the section width.6 In 285/70R17, the sidewall height is 70% of 285 mm. That is 199.5 mm, or 7.85 in.7 A lower aspect ratio gives a shorter sidewall.
Construction: the letter R
The letter R shows radial construction.1 FMVSS No. 139 tells the manufacturer to put the word RADIAL on the sidewall of a radial tire.8
Rim diameter: the last number of the size
The last number of the size is the rim diameter in inches.1 A 285/70R17 tire is for a 17 in rim.
Overall diameter: the number that is not on the tire
The size does not show the overall diameter. You can calculate it from the three numbers.7
- Multiply the section width by the aspect ratio. Then divide by 100. For 285/70R17: 285 × 70 ÷ 100 = 199.5 mm.
- Divide the result by 25.4 to get inches: 199.5 ÷ 25.4 = 7.85 in.
- Add two sidewalls to the rim diameter: 17 + 2 × 7.85 = 32.71 in.
The result is the nominal diameter. A real new tire can be a little smaller or larger. For this size, UN R30 permits a radial tire from 32.24 in to 33.34 in.9 Use the calculator to get these numbers for any size.
Load index and speed rating
The service description comes after the size. In 121/118S, 121 and 118 are load indexes and S is the speed rating.10
The load index is a code for the load that the tire can carry.11 Load index 121 is 1,450 kg (3,197 lb).12 The second number is the load for each tire when two tires are side by side (dual). Load index 118 is 1,320 kg (2,910 lb).13
The speed rating is the speed at which the tire can carry the load of its load index.14 S is 180 km/h (112 mph).15 The pound and mph values are our conversions of the official kg and km/h values.
The US list of necessary sidewall markings has the maximum load rating, but not the load index or the speed rating.16 Many manufacturers add them to the size.
Other marks near the size
- The letters DOT and the tire identification number (TIN). The last four digits are the date code.17 See how to read the date code.
- The maximum inflation pressure.18 This is not the pressure for your vehicle. Use the pressure on the vehicle placard.19
- On LT tires, the load range letter, for example E.20 See load range and ply rating.
- On passenger tires, the UTQG grades for treadwear, traction and temperature.21 See UTQG ratings.
- On a tire that meets the definition of a snow tire, the manufacturer can add a mountain and snowflake symbol.22
Some truck tires use inches, for example 33x12.50R15LT. See inch tire sizes.