Getting a grip on tire fundamentals

A tire's sidewall is packed with information. Each manufacturer's Web site has a detailed key translating the figures, but this quick primer will help you decipher the information. Look for an alphanumeric combination such as "P245/40R17 91Y." The "P" indicates the tire was designed to meet North American passenger-car standards. If it were "LT," the tire was designed for light trucks. A lack of a preceding letter means a tire designed for European standards. The "245" is the nominal section width (not exactly the same as tread width but close) in millimeters. The "40" is the aspect ratio: The sidewall height expressed as a percentage of the tread width. In this case, 40 percent of 245 mm, or 98 mm. "R" indicates a traditional radial tire design. "17" is the wheel diameter in inches. "91" is the load index-a larger number means the tire, if inflated properly, can carry more weight, in this case 1,356 pounds. "Y" is the speed rating, meaning this tire is capable of sustained speeds of 186 mph. Other common speed ratings are S = 112; T = 118; H = 130; V = 149; W = 168; Z greater than 149.

112 0504 Ic Tire Side Z Tire manufacturers have a size "window" in which to fit a nominal dimension. While all of Maker A's 245/40R17s will be identical, they may be ever-so-slightly wider, narrower, shorter, or taller than Maker B's 245/40R17s. A tire focused on performance will likely be in the wide and tall corner of the window, while one that prioritizes low cost will likely be in the narrow and short corner. Since the size windows overlap, some makers use one size mold to produce a pair of dimensions. "M+S" means the tire meets the standards for an all-season tire. This does not imply the tire performance rivals a dedicated snow tire, just that it's better in snow than one without the M+S designation.

Widely misunderstood, "Max Load/Max Pressure" is not the pressure at which the tire will burst. Instead, increasing pressure beyond this point results in no additional load-carrying capacity beyond that stated.

Uniform Tire Quality Grade is shown as numbers or letters after the words "treadwear," "traction," and "temperature," this is the tire manufacturer's self-assessment of how this tire compares to its other offerings. The treadwear number, determined by field testing on a government-specified course, is a percentage of a control tire graded at 100 points. Thus, a tire marked "200" means it lasts twice as long one graded "100." The traction rating comes from a straight-line wet (more like damp) grip test (not really stopping distance, as its performed on a traction trailer). "AA" is best. And temperature indicates the tire's resistance to heat generated by running at high speed. "A" is best. So many high-quality tires get "AA" and "A" in the last two categories, that they are of relatively little use, except to convince you not to buy cheap tires.

Courtesy of MotorTrend.com