First, experts say you should understand the kind of tire you need. They caution that you should stick with the size, type and load rating of your original-equipment tires. Because SUVs and light trucks are more likely to be overloaded than cars, load ratings are especially important.
Choose a tire type and size to suit your vehicle and driving conditions:
A popular trend is to "plus size" tires and wheels. Drivers buy larger wheels and shorter, wider tires. For example, if your car specifies a wheel size of 16 inches, increasing wheel size by 1 inch would result in a 17-inch wheel or "plus one" for your vehicle. The tire you buy will then need to have a shorter sidewall, and usually a wider tread, to accommodate the same load and tire diameter.
Although many drivers are doing this for cosmetic reasons -- a larger wheel and lower-profile tire have a sportier look -- some experts say you can gain traction and handling by plus-sizing, because a wider tire can get a better grip. ConsumerReports.org says plus one gains you the most benefit at a lower price; larger plus two or plus three wheels and wider tires cost exponentially more and offer less performance gain over plus one. They also add some risks, ConsumerReports.org says:
There are many considerations to factor into a decision to plus size your tires. First, your vehicle must be able to accommodate the additional offset of a wider tire. "Be wary of spacers, which retailers often sell as a way to make wheels fit by moving them out from the hub," ConsumerReports.org says. Also, you must be sure that the tire's load rating is at least the same as your factory-specified tire; the tire may require a pressure change to achieve that. The new wheels should be made specifically to fit your vehicle, to prevent strain on the lug nuts and bolts. Experts advise buying from a dealer/installer with experience in plus-sizing who can identify the appropriate size options for your vehicle.
Passenger tire sizes, which are called P-metric, and light truck tire sizes, also known as LT-metric, are represented by a ratio followed by a whole number. The ratio is the width in millimeters over the percentage of that width that makes up the profile (the distance between the rim and the outer edge of the tire); the whole number is the rim diameter in inches. A tire with a size of 215/60-16 is 215 millimeters wide, 129 millimeters from the ground to the rim (60 percent of 215) and is intended for a 16-inch wheel.
Tire manufacturers use letters to represent the maximum speed at which a tire should be driven. There are about 20 speed ratings, but the most common for SUV and light-truck tires are Q (up to 99 mph), S (up to 112 mph), T (up to 118 mph) and H (up to 130 mph). Experts say that even if you have no intention of driving at speeds of 130 mph or more, tires with a high speed rating are also better performers at more mundane speeds.