*For tires having a maximum speed capability above 149 mph, a ZR may appear in the size designation. (Of course, you should always abide by speed limits for safer driving.) Speed rating is usually, but not always, a single letter (see the chart).īelow you will find several charts that will help you understand tire sizing numbers, including a load index chart and speed rating chart. The maximum operating speed of a vehicle is no more than the lowest speed rating of all tires mounted on the vehicle. A tire with a higher speed rating can handle heat better and provide more control at faster speeds. This indicates the top speed it’s safe to travel at for a sustained amount of time. G: SPEED RATING The last letter is the tire speed rating. In our example, the load index is 89, which indicates the tire has a load capacity of 1,279 pounds, when inflated to the tire’s maximum air pressure rating. The load index symbol indicates how much weight a tire can support, based on the following standard chart. It’s the distance between the two bead seat areas (where a tire gets tightly sealed onto the wheel).į: LOAD INDEX The two-digit or three-digit number that follows the gap specifies tire load index. Some run-flat tires are identified with an F followed by the type of internal construction.Į: WHEEL DIAMETER This two-digit number specifies wheel diameter in inches. Some motorcycle and trailer tires still use this internal construction. They are also called conventional, x-ply, or cross-ply tires. In a radial tire, the plies - layers of strong cords made of a blend of polyester, steel and fabric and coated with rubber - are laid perpendicular to the direction of travel.ĭ is for tires built with diagonal (crisscrossed) plies, called bias-constructed tires. They have better road grip, lower rolling resistance for better gas mileage, ride comfort and durability than previous generations of tires. R is for radial tires, the industry standard for most tires today. This gives you an answer of 5.5, the sidewall height in inches.ĭ: CONSTRUCTION TYPE This single letter tells you about the internal construction of the tire. To get the sidewall height, take the tire width of 215 mm and convert it to inches (8.46). In this example, the aspect ratio is 65, meaning the sidewall is 65 percent as high as the tire is wide. It’s the height of the sidewall measured from wheel rim to top of the tread, expressed as a percentage of tire width. The aspect ratio is indicated on the tire sidewall as a percentage. The bigger the aspect ratio, the higher/taller the tire’s sidewall, or “profile” as it’s sometimes called. This may also be referred to as the section width.Ĭ: ASPECT RATIO The forward slash separates the tire width number from the two-digit aspect ratio. It’s also measured in millimeters but may have a different load capacity than a P or LT tire.ī: TIRE WIDTH The three-digit number following the letter is the tire’s width (from side to side, looking at the tire head on) in millimeters. If there’s no letter before the first number, you have a metric tire most commonly referred to as European size. These tire sizes are meant for trailers, including fifth wheels and other travel trailers, as well as boat and utility trailers. These are often equipped on three-quarter or 1 ton trucks and SUVs. LT means light truck tire, designed for vehicles that are capable of carrying heavy loads, towing trailers, or for those looking for an extra heavy duty option. P-class tires include cars, SUVs, crossovers, minivans and smaller pickup trucks.
#Tire size conv how to
Here’s what those numbers and indicators on the sidewall indicate and how to understand them:Ī: TIRE TYPE The first letter in the code tells you what class of tire it is.
#Tire size conv professional
Next, have a tire professional help you determine a tire size range that will fit your vehicle and driving needs. If you’re interested in switching out your tires for a different look or performance, a good place to start is the numbers and other indicators on your existing tires’ sidewall. This is the sizing recommended by the vehicle manufacturer.
#Tire size conv manual
You can see your original equipment tire size in your owner’s manual or on the placard generally located on the driver’s side door jam. Plus, the right size for your car, truck, or trailer can differ depending on where and how you drive.
Some numbers on the sidewall are listed in millimeters while others are inches. Tire Size Explained: What the Numbers Mean
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