Gearhead vocabulary is a colorful collection of terms that’s nearly impossible to understand for the flatheads out there who don’t talk the talk. But how many of us are fluent in “rubberese”? Truth is tires speak a language all their own, and the ability to decipher those numbers and letters branded around the sidewall is even more important than knowing your stacks from your steelies.

As the only part of your vehicle actually touching the ground (normally), it’s important to know your tires’ capabilities and limitations. In fact, your ride will only perform as well as the tires mounted underneath it: acceleration, braking, handling, and safety are all dependent on your tires first and the mechanical bits second.

Using a P 245/55R18 90W tire as our example, let’s crack the code:

Tire Class/Service Type

P 245/55R18 90W

On most tires, the first letter (or letters) indicates its intended vehicle type or usage. In this case, the code begins with a “P,” denoting that this tire was designed for use on Passenger Vehicles, such as cars, minivans, and light trucks/SUVs. Tires beginning with “LT” are for use exclusively on Light Trucks (including full-size trucks, SUVs, vans).

Section Width

P 245/55R18 90W

Measured in millimeters, this number indicates the width of the tire from sidewall to sidewall, excluding any lettering or designs. In this example, the tire is 245 millimeters (9.65 inches) wide.

Aspect Ratio

P 245/55R18 90W

Aspect Ratio is the height of the sidewall, as a percentage of the section width. In this example, the number 55 tells us that the height of the sidewall is 55 percent of the section width. To determine the height, simply multiply 245mm by 55 percent (.55). The tire’s sidewall is 134.75mm (5.30″) tall.

Internal Construction

P 245/55R18 90W

Radial Tires, marked with an “R,” are the most popular type, representing about 98 percent of all tires manufactured. Here, plies are arranged so the cords in the body run at a 90-degree angle to the centerline of the tread. The result is a more flexible tire with minimal rolling resistance.

Bias Ply Tires, marked with a  “D,” feature a diagonal or “bias ply” construction, in which the cords crisscross the centerline of the tire at a 32 to 45-degree angle. This construction method is used for high aspect-ratio applications, like off-road truck, drag racing, PowerSports, and reproduction tires.

Important Notes: We do not recommend mixing radial and bias ply tires on the same vehicle. If you must, always position the radials on the rear axle. Never mix radial and bias ply tires on the same axle. When replacing two tires, we recommend you position them on the rear axle. When replacing a single tire, it should be positioned on the rear axle, paired with the tire having the most tread depth of the remaining three.

Tire/Wheel Diameter

P 245/55R18 90W

The number 18 is the “inch rim” size. This denotes the inside diameter—in inches—of the tire, as well as the appropriate wheel diameter. Traditionally, these are whole numbers; however, some trailer and off-road truck tires come in half-inch sizes to fit specially designed wheels.

Load Index

P 245/55R18 90W

The load index refers to the amount of weight a tire can support. As the load index increases, the maximum weight increases accordingly. Typical passenger vehicle tires have a load index of 70 to 110 (761-2,337 pounds.). With a load index of 90, this tire can support 1,323 pounds.

Speed Rating

P 245/55R18 90W

The final letter is the speed rating—a universal system originally developed as a way for German drivers to select tires for safe, high-speed autobahn highway travel. The speed rating, denoted by the letters M through Z, indicates the maximum speed at which the tire can safely travel. In this case, the tire is “W” rated, telling us that it’s been tested to safely travel up to 168 mph.

Here’s a rundown of all tire speed ratings:
M: 81mph or 130 km/h
N: 87 mph or 140 km/h
P: 93 mph or 150 km/h
Q: 99 mph or 160 km/h
R: 106 mph or 170 km/h
S: 112 mph or 180 km/h
T: 118 mph or 190 km/h
U: 124 mph or 200 km/h
H: 130 mph or 210 km/h
V: 149 mph or 240 km/h
Z: in excess of 149 mph or 240 km/h
W: 168 mph or 270 km/h
Y: 186 mph or 300 km/h

Note that some tires include the Z speed rating within the size designation (example: P 255/50ZR18 W). Prior to 1991, Z was the highest speed rating given to tires, indicating that they had been tested to exceed 149 miles-per-hour. Manufacturers commonly add this designation in addition to W or Y ratings.

Also, it’s important to remember that just because your tires are rated to 149 miles-per-hour, that doesn’t mean that your vehicle can safely travel at those speeds. Consult your owner’s manual for the proper tire class, size, and load index, and speed rating.

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Author: Dan Michaud