 We all know that the numbers...  We all know that the numbers on the side of the tire determine its size, but how do you convert these numbers to a usable measurement? In this section we'll define what the P-metric numbering system means, and show you how to convert the numbers to usable dimensions for fitting tires to your car. |
 Rim diameter is the diameter...  Rim diameter is the diameter of the rim where the inner circumference of the tire seats, not the rims maximum diameter. A wheel will nearly always measure an inch larger than the tire it will accommodate at its maximum diameter. Shown here is a 15-inch police car wheel that measures 16 inches at its maximum diameter. |
 Rim width is the distance...  Rim width is the distance between the inside of the lips where the beads of the tires seat and is an important consideration when choosing tires. There should be no more than 1-inch difference between the tire's tread width and the width of the rim. |
 Using a straight edge from...  Using a straight edge from the axle flange, the required backspacing, rim width, and tread or section width can all be measured with a tape measure. Be sure to take multiple measurements around the circumference of the wheelwell to ensure your wheel or tire won't have any interference problems. |
P275/60R15
Modern tires are sized using the P-metric format and have a letter or series of letters before the first numbers on the tire. These letters designate the intended use of the tire. P is for passenger vehicles, LT for light trucks, and ST for special trailer service. If a tire's numbers are followed by a C, it designates the tire is for commercial use and will usually bear a higher load rating than standard tires.
P275/60R15
Following the letters on the tire is a series of three numbers indicating the tire's section width in millimeters. The section width is the maximum width of the tires cross section, or sidewall-to-sidewall, when mounted to a rim of a specific size. Converting the section width number to inches can be accomplished easily by dividing the number of millimeters in the section width by 25.4 to obtain the dimension in inches. Using the same example of a P275/60R15 tire, dividing the 275mm section width by 25.4 gives us a section width of 10.82 inches. This will be the maximum width of the tire from sidewall-to-sidewall, but does NOT take into consideration any raised lettering on the tire, so be sure to add the appropriate amount to the section width if your tires have raised letters.
P275/60R15
The next two digits on a tire's sidewall typically follow a slash mark and define the tire's sidewall aspect ratio. This number indicates, in a percentage, the tire's sidewall height from rim to tread as it relates to the section width of the tire. In our P275/60R15 example, the tire's sidewall height would be 60 percent of its section width. Since we know the section width is 275 mm, or 10.82 inches (275 divided by 25.4), multiply that number by .60 (sixty percent) to get a sidewall height of 165 mm, or 6.49 inches. Since there is a sidewall at both the top and bottom of the tire, by doubling the sidewall (6.49x2) and adding it to the rim diameter (15), we can derive the total height of the tire (6.49x2+15) = 27.98 inches. The height of a tire, in inches, can also be derived from its P-metric code by using the following formula: H (height) = (Wx(ratio/100)x.03937x2) + rim diameter. W is the tire's section width in millimeters, and rim diameter is in inches. By plugging in our P275/60R15 example we do the math as follows: H = (275x(60/100 x.03937x2) + 15. Using a calculator, we derive the tire height to be 27.99 inches. This factor will work for any P-metric tire and is a useful way to determine the overall height of P-metric tires in inches, which is the way we measured our car for tire fitment.
P275/60R15
The letter following the aspect ratio on the sidewall of the tire simply has to do with the construction of the tire. "R" indicates the tire is of radial (R) construction, that is, the tire's body plies "radiate" out from the imaginary center of the wheel. This is by far the most common type of tire construction representing over 98 percent of all tires sold today. If a "D" is found in this position, it indicates the tire's plies crisscross on a diagonal (D), and the tire is a bias-ply construction. These tires are typically used on light-duty trucks or for spare tire applications. A "B" in this position identifies that the tire not only has crisscrossed plies, but that it also is reinforced with belts under the tread area. This type of construction is called belted (B) and is nearly extinct. If a "Z" is found in this area before the construction designation (P275/60ZR15) it indicates the tire has a "Z" speed rating. Only tires with a "Z" rating will have the rating indicated on the sidewall.
P275/60R15
The final two-digit code on the sidewall indicates the rim size the tire is designed to fit, in our case a 15-inch rim.