Home Automotive News What do the numbers and letters mean on tires?

What do the numbers and letters mean on tires?

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General-Tire-Cover-PicReading Automotive Tires

Tires receive ratings by the U.S. Government through meeting minimum standards for reaching and sustaining a specified speed & load ratings

P means this is a passenger-car tire. Other codes include T (temporary, as in a spare tire), LT (light truck) and C (commercial).

Example P205/55-R-17 87

Example 205 (the first number) is the tire’s width in millimeters.

Example 55 (the number following the slash) is a measure of the tire’s profile — the height of its sidewall relative to its width — expressed as the sidewall’s aspect ratio in a percentage. In this example, the sidewall height is 55 percent of the tire’s width, which is 205 mm. The number is known as the tire’s series; a sporty tire with shorter sidewalls would be considered a lower-series tire. Off-road tires tend to be higher-series, designed to absorb the impact from rough surfaces.

R means it is a radial tire, which describes the way the underlying layers, or plies, are laid out.

Example 17 is the rim size

Example P205/55-R-17 87

The load index (87) is the tire size’s assigned numerical value used to compare relative load carrying capabilities. In the case of our example the 87 identifies the tires ability to carry approximately 1,201 pounds. The higher the tire’s load index number, the greater its load carrying capacity.

If the tire has M+S stands for mud and snow, indicating that this is an all-season tire.

Other Tire Info

Uniform Tire Quality Grade

The following three codes, which appear on the sidewall, typically closer to the tire’s tread, are part of the National Highway Traffic Safety Administration’s uniform tire-quality grading system. Unfortunately, it’s not all that uniform: It appears on all-season and summer tires, but not on snow tires, light-truck tires or spares. Further, the rating comes from the manufacturers, whose results might be consistent from one of their products to the next, but whose universality is questionable.

TREADWEAR 300 TRACTION A TEMPERATURE A

Treadwear: Uses numbers from 100 to about 700. Theoretically, a tire rated 150 would have a 50 percent longer lifespan than a tire rated 100 if used in the same conditions — driver, vehicle and roads. Soft summer tires tend to have lower treadwear ratings than all-season tires. Winter tires aren’t rated, but their soft compounds make for quick wear when driven off snow and ice.

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The uniform tire quality grade is located on the sidewall near the tread.

Traction: Uses AA, A, B and C, with AA being the best traction on a wet road. The traction measured is straight-line acceleration and braking. This is not a measure of cornering grip or performance on dry surfaces.

Temperature: Uses letter grades A, B and C, with A representing the best resistance to heat buildup and C the least. The friction of a tire on pavement generates heat, and too much heat degrades high-speed performance and can accelerate aging and failure. C is the lowest permissible rating. Temperature ratings correlate to speed, with C representing 85-100 mph, B representing 100-115 mph, and A covering speeds above 115 mph.

Tire Speed Ratings

On tires speed ratings are designated by letters note the following chart.

B – Up to 31 mph

C – Up to 37 mph

D – Up to 40 mph

E – Up to 43 mph

F – Up to 50 mph

G – Up to 56 mph

J – Up to 62 mph

K – Up to 68 mph

L – Up to 75 mph

M – Up to 81 mph

N – Up to 87 mph

P – Up to 94 mph

Q – Up to 100 mph

R – Up to 106 mph

S – Up to 112 mph

T – Up to 118 mph

U – Up to 124 mph

H – Up to 130 mph

V – Up to 149 mph

W – Up to 168 mph

Y – Up to 186 mph

Z – 149 mph and over

Speed Rating Note

Speed Rating For Cars that can travel over 149mph

When Z-speed rated tires were first introduced, they were thought to reflect the highest tire speed rating that would ever be required, in excess of 240 km/h or 149 mph. While Z-speed rated tires are capable of speeds in excess of 149 mph, how far above 149 mph was not identified. That ultimately caused the automotive industry to add W- and Y-speed ratings to identify the tires that meet the needs of new vehicles that have extremely high top-speed capabilities.

W

168 mph

270 km/h

Exotic Sports Cars

Y

186 mph

300 km/h

Exotic Sports Cars

While a Z-speed rating still often appears in the tire size designation of these tires, such as 225/50ZR16 91W, the Z in the size signifies a maximum speed capability in excess of 149 mph, 240 km/h; the W in the service description indicates the tire’s 168 mph, 270 km/h maximum speed.

225/50ZR16

in excess of 149 mph, 240 km/h

205/45ZR17 88W

168 mph, 270 km/h

285/35ZR19 99Y

186 mph, 300 km/h

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