Having the correct tire size is not only important for handling and performance; it also helps keep you safe. Your vehicle is designed for all parts to work together, and tires are a crucial element — some say the most crucial because they’re the only parts of your vehicle that actually touch the road, affecting not only handling but acceleration and braking forces.
Decoding the digits. Besides the tire’s brand name and model, the sidewall of your tire tells you a lot about its size and characteristics, including its age and intended uses. The basics of deciphering the code go like this: Find the size designation, which will look something like this: 215/60-15R. The first three digits state the width of the tire, from outer sidewall to inner sidewall, in millimeters (for inches, divide by 25.4). The next two digits after the slash describe the height of the sidewall, from rim to tread, as a percentage (in this case 60%) of the width. This is called the tire aspect ratio, also sometimes referred to as the “profile,” or “section.” After the aspect ratio and hyphen, the next two digits describe the wheel or rim diameter the tire will mount to, in inches. Our example is for a 15-inch wheel. The “R” means the tire is a radial. There are still some belted tires for trucks (designated B) and bias-ply tires for older vehicles (with a D, for the diagonal orientation of the belt). If you need more details, see the National Highway Traffic Safety Administration’s advice here: http://www.nhtsa.gov/cars/rules/tiresafety/ridesonit/brochure.html
Tire sizes also affect the accuracy of your speedometer. Speedometers are hooked up to the transmission and calibrated for the revolution of the recommended tire size. A taller or shorter tire will cause inaccurate readings. If you’re changing wheel diameter for appearance or performance reasons, the new tires should be of a size that keeps the total diameter and circumference roughly the same — so a bigger wheel needs a tire with a shorter sidewall. That means a numerically lower aspect ratio, perhaps a 40- or 50-section instead of a 60, for instance.
If you’re still unsure how to read tire size or understand tire size conversion, check your owner’s manual, or use any of the resources provided to you by Jiffy Lube®.