How Are Tires Rated?
Passenger and light truck tires are subject to several rating systems that allow buyers to select tires that meet their requirements. Of extra help is the fact that all of the rating data is molded right into the sidewall of the tire. This allows the shopper to easily compare tires side by side or find a rating match to the tires already on the car.
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Tire Load Rating
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All tires have a maximum load capacity; this is the most weight the tire can safely carry at the maximum rated tire pressure. A passenger tire's load rating is printed on the tire sidewall after the tire size and along with the speed rating. The load and speed rating will look like 92S or 100H, respectively. The number part represents the load rating and range range from 60 to 125. Passenger car tire ratings range from 70 to 110 and light truck tires can have higher load limits. A tire with a 70 rating will support a load of 738 pounds and a 110 rated tire can carry 2,337 pounds. It is important to fit a vehicle with tires that have a load rating equal to or higher than the original equipment tires.
Light Truck Tire Load Ratings
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Light truck tires will also have letter ply rating/load range designation. Light truck tires will have an LT at the beginning of the tire size and a letter B through F at the end of the size listing. The letters list the tire's ply rating. Ply ratings are based on the pre-radial construction method of adding more belts of fabric to make tires stronger. The ply rating also indicates the maximum pressure to which a tire can be inflated. Here are the ply ratings, ply equivalent and maximum inflation pressure:
--B, 4-ply, 35 psi
--C, 6-ply, 50 psi
--D, 8-ply, 65 psi
--E, 10-ply, 80 psi
--F, 12-ply, 95 psiThe tire manufacturer's data book must be checked for the maximum load on these tires.
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Speed Rating
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A tire's speed rating is indicated by the letter after the load rating numbers. Speed rating letters are L,M,N,P,Q,R,S,T,U,H,V,W,Y in order from slowest to fastest. The speed ratings were developed in Germany and start at 120 kilometers per hour for L and step up in 10-kph increments to H at 210 kph. V,W and Y indicate tires rated for 240 kph, 270 kph and 300 kph, respectively. Commonly used speed ratings converted to miles per hour are S at 112 mph, T at 118 mph, H at 130 mph and V at 149 mph. Exotic sport cars with W- and Y-rated tires have rubber with speed ratings of 168 and 186 mph, respectively.
Wear Ratings
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The Uniform Tire Quality Grade standards (UTGQ) ratings provide wear and traction information for each tire. The wear rating tests a tire under controlled conditions for 7,200 miles and compares the wear against a reference tire. A tire that has the same rate of wear as the reference receives a score of 100. A 200 grade means the tire would last twice as long as the reference tire. The wear rating tests are conducted by the tire manufacturers themselves. Comparing wear rating grades between a single manufacturer's tires will give an accurate picture of the relative life of the tires. Comparing between manufacturers may not be as accurate.
Traction Rating
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Another portion of the UTGQ score is the traction rating. Tires are tested for there coefficient of traction in wet conditions at 40 mph. Tires are given a letter grade based on the coefficient of friction that is measured for the tires. The ratings from most wet traction to the least are AA, A, B and C, respectively. All new tires must have a wet traction score of at least C.
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References
- Photo Credit tire image by Orlando Florin Rosu from Fotolia.com
