Comparing tire sizes is easy once you understand the various marking on the tire’s sidewall but keep in mind that there is more to understand when buying tires than just their size.
Look at the sidewall of your car’s tires for a sequence of numbers and letters that looks something like this: P235/75R15 85H.
The letter “P” indicates that the tire was designed for passenger cars. If the “P” is replaced by an “LT” it indicates that the tire was designed for light trucks.
The “235” is the section width, also known as the tread width, of the tire in millimeters.
The “75” is the aspect ratio of the tire and probably the one component of tire size that more people have trouble understanding than any other. Simply put it is the percentage of a tire’s height above the rim vs. its width. In this example the tire is 235 millimeters wide and it’s height above the rim is 75% of that or 176.25 millimeters. A tire with a lower aspect ratio will tend to handle better than one with a higher aspect ratio but will tend to have a harsher ride.
The “R” indicates that the tire is a radial ply tire but almost every passenger car tire made today uses radial ply construction. If you are purchasing a bias ply tire for some reason the “R” would be replaced with either a “D” for diagonal bias or “B” for belted bias.
The “15” is the diameter of the rim in inches. This is a measurement that has to stay the same as the tires which originally came on your car unless you also replace the wheels. A 14 inch tire for example is too small to fit over a 15 inch rim and a 16 inch tire is too big to allow the bead to set correctly on the rim.
The “85” is the load rating of the tire with a higher number indicating the ability to carry a heavier load. Because the load rating is closely matched to the weight of the car plus the anticipated weight of any passengers and cargo you should never purchase a tire with a lower load rating than what the car was originally equipped with. Some tires, especially light truck tires, may not have this number or the speed rating listed on the sidewall.
The “H” is the speed rating for the tire and indicates the maximum speed at which a tire is engineered to operate. Most passenger tires have speed ratings of: “S” (112 mph), “T” (118 mph), “U” (124 mph) or “H” (130 mph). High performance tires will typically have a “V” (149 mph) or “Z” (Above 149 mph) speed rating. Dedicated snow tires or spare tires will usually have a lower speed rating.
