Tire School

The tires above are the two types that are available on a 2009 G37 Coupe. The top one, a P255/45 R19 92W,  is part of the sport package, and is also standard on vehicles equipped with a manual transmission. The bottom one is a P245/45 R18, standard equipment on the automatics without the sport package. What do all the letters and numbers mean ? Well, here goes...

Most tire sizes begin with a letter or letters that identify the type of vehicle and/or type of service for which they were designed. When a tire size begins with a "P," it signifies the tire is a "P-metric" size that was designed to be fitted on vehicles that are primarily used as passenger vehicles. This includes cars, minivans, sport utility vehicles and light duty pickup trucks (typically 1/4- and 1/2-ton load capacity). The use of P-metric sizes began in the late 1970s and they are the most frequently used type of tire size today.

Following the letter that identifies the type of vehicle and/or type of service for which the tire was designed, the three-digit numeric portion identifies the tire's "Section Width" (cross section) in millimeters. With a
P225/45 R19 92W for example, the 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tire's section width. Because many people think of measurements in inches, the 225mm can be converted to inches by dividing the section width in millimeters by 25.4 (the number of millimeters per inch).

Typically following the three digits identifying the tire's Section Width in millimeters is a two-digit number that identifies the tire's profile or aspect ratio. In our P225/45 R19 92W for example, the 45 indicates that this tire size's sidewall height (from rim to tread) is 45% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall.

The R in the P225/45R19 92W size identifies that the tire has a Radial construction in which the tire's body plies "radiate" out from the imaginary center of the wheel. Radial tires are by far the most popular type of tire today representing over 98% of all tires sold.

The 19 indicates the tire and wheel diameter designed to be matched together. Tires that have a rim diameter expressed in inches (P225/45R19, as well as 8, 10, 12, 13, 14, 15,16, 17, 18, 20, 22, 23, 24, 26 and 28) are called "inch rim" sizes, are the most common type of tire size and are used on most cars, minivans, vans, sport utility vehicles and light duty light trucks.

The 92W at the end of the size represents the tire's service description. A service description identifies the tire's load index and speed rating. Service Descriptions are required on all speed rated (except for Z-speed rated) tires manufactured since 1991.

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 rated tires are rated to 168 mph.

And now you know your tires. For more information, check with the experts in the Harte Infiniti Parts Department !!

Source: Tirerack.com