How to read tire size
Upgrading your vehicle’s wheels and tires is a popular modification that can improve the look and performance of your ride, and picking the right tire can be easy—if you know where to look.
The first step is to determine what size wheels and tires you currently have and then decide how big or small you want to go. A size is always shown on the sidewall of any tire. Passenger vehicle tires are usually in metric form and larger truck tires can be shown as metric or standard size.
This diagram explains what each number or letter means.
|1. PrefixP = Passenger car
T = Temporary spare
LT = Light truck
ST = Trailer
|2. Section widthThat`s the overall width of the tire in millimeters when mounted on the correct width rim.|
|3. Aspect ratioThis indicates the distance from the wheel-rim mounting surface to the outside of the tread. Aspect ratio is expressed as a proportion of the width instead of as an absolute number, so it’s easier to compare the profiles of different-size tires. A 50-series tire is half as tall as wide, while a 70-series is 70 percent as tall as its width.||4. Speed ratingOnly the highest-rated tires, the Z-rated models, still label the speed rating inside the tire-size designation. On all other tires, it’s found elsewhere on the sidewall.|
|5. ConstructionR = Radial-ply tire.
D = Refers to diagonal, or old-fashioned bias-ply tire, now used only on trucks and trailers.
|6. SizeThe diameter of the rim in inches. If it seems odd to you to spec tire width in millimeters and diameter in inches, it does to me, too. A few special-purpose tires, like PAX system tires, use a metric designation for rim diameter.|
A standard sized tire will be displayed in inches. A tire that reads 33 x 12.5 R15, is 33-inches in diameter (or tall), 12.5-inches wide, and will fit a 15-inch wheel.
The door jam of your vehicle will also indicate what size tires the manufacturer recommends. Going beyond that size will throw off your gearing and result in slower acceleration. It will also change your speedometer reading. Most tire shops will have a maximum wheel and tire size listed for your vehicle that will still fit without rubbing, but it’s always best to check multiple sources and take advantage of online forums for advice.