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There are a few specifications on automotive tires that are important for consumers to be aware of. Some of these specifications are not negotiable, like the size of the tire. The ones an average consumer should be aware of are size, load, speed, tread wear, traction, and pressure.
Automotive tires come with a series of numbers on them. These numbers indicate the various measurements of that particular tire. For example, a tire may read: 245/65/17 105H.
The first three numbers represent the size of the tire. The first number, 245, is the width of the tire in millimeters. The second number, 65, tells the aspect ratio. In this example, the 65 means that the height is 65% of the width. Multiply 0.65 X 245 to find that the height is 159.25 mm. The third number, 17, indicates the rim diameter. The size of tire a car left the manufacturing plant with is the size that must be on the car as long as no major modifications have been done to the vehicle.
The second half of the numbers on automotive tires list the load index and speed ratings. Load tells how much a vehicle can haul, a measurement that is of particular interest to those with trucks. The load index has to be looked up on a special load index chart. In this case, 105 means that it can hold 2,039 lbs (924 kg) per tire. Multiply 2039 x 4 to find that the maximum load capacity is 8,156 lbs (3,700 kg).
The final letter represents the speed ratings, or how fast a tire can travel before it will blow out. Ratings are: S-112 mph (180 kph), T-118 mph (190 kph), U-124 mph (199 kph), H-130 mph (209 kph), V-149 mph (240 kph), W-168 mph (270 kph), Y-186 mph (299 kph).
The tread wear rating on automotive tires tells how thick and strong the tread is. The higher the tread wear rating, the longer a tire will last. The ratings range from 100 up to 600. The base rating is 100, while all numbers above that show how long the tire will last compared to that base. A tire with a rating of 600 is expected to last six times longer than a base tire.
The traction rating gives an idea of how automotive tires will handle on various terrains. Some tires are rated for snow and mud in particular. For those in a snowy or rainy climate, it is essential to be sure to purchase "four season" tires.
The final measurement consumers should be aware of is the pressure rating. Automotive tires with a higher peak pressure tend to get better gas mileage as do those with a higher rated tread wear. The belief is that the harder surfaces mean less resistance on the road, and so less effort to move forward.