How to read a tire size

Use LT275/65R20 126/123S alongside the sidewall drawing. The size portion describes the tire type, nominal width, aspect ratio, construction and rim diameter. The service description follows it.

By the formula, this size measures 34.1 in (866 mm) in overall diameter, with a sidewall height of 7.0 in (179 mm). Its nominal circumference is 107.0 in (2,719 mm), giving 592 revolutions per mile (368 per km). These are calculated dimensions, not measurements published by a tire maker.

• LT: light truck tire.
• 275: nominal section width in millimeters, measured from sidewall to sidewall.
• 65: aspect ratio; sidewall height is 65% of section width.
• R: radial construction, not rim diameter.
• 20: rim diameter of 20 in (508 mm), not overall tire diameter.

The pair 126/123 gives the load indexes for single and dual mounting. Toyo's published tables list 126 as a maximum of 3,748 lbs (1,700 kg) per tire and 123 as 3,417 lbs (1,550 kg) per tire. These codes describe load capacity, not tire dimensions.

Toyo lists S as 112 mph (180 km/h). Toyo explains that the speed symbol identifies the speed at which a tire can carry its indexed load under the maker's specified service conditions. Read the load index and speed symbol together.

49 CFR 574.5 specifies a tire identification number, or TIN, consisting of 13 symbols for a new tire. Its final four digits identify the week and year of manufacture. Locate that final group when reading the tire date code, rather than treating the whole identification string as a date.

49 CFR 571.139 (FMVSS No. 139) requires the maximum load rating and maximum permissible inflation pressure on the sidewall. The pressure marking is a limit, not the pressure to run. For inflation pressure, see the tire and loading placard on the driver's door jamb.

49 CFR 575.104 (UTQG) covers passenger-car tire grades, with exclusions including deep-tread winter snow tires and temporary spares. Bridgestone says M+S or M/S indicates some capacity to handle mud and snow. The mountain-snowflake symbol is a separate marking: under 49 CFR 571.139 (FMVSS No. 139), a tire meeting the snow tire definition may carry it at the maker's option.

• UTQG treadwear: a numerical grade expressed in multiples of 20.
• UTQG traction: AA, A, B or C.
• UTQG temperature: A, B or C.
• Mountain-snowflake: the snow tire definition requires a traction index of 112 or more against the ASTM F2493 reference tire.

Bridgestone identifies P as the P-metric passenger-car tire type and LT as the LT-metric light truck tire type. Bridgestone describes light truck tires as intended for vehicles capable of carrying heavy cargo or pulling trailers. The prefix identifies a tire type; it is not another dimension.

49 CFR 571.139 (FMVSS No. 139) requires a load range letter on LT tires and separately requires actual ply counts. The load range letter is therefore separate from the actual number of plies. Under 49 CFR 575.104 (UTQG), LT tires do not carry UTQG grades.

Nitto lists the Ridge Grappler in LT275/65R20 with load range E and service description 126/123Q. Nitto lists its overall diameter as 34.09 in and inflated overall width as 10.98 in, illustrating why maker-listed dimensions should be distinguished from nominal calculations. Fit depends on the vehicle, wheels and suspension, not just the size string.