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Tire Talk Reading Your Tires Sidewalls

Tire Talk Reading Your Tires Sidewalls Hamilton

Tires Come In All Sizes

Zoomy, stove bolt, T-bucket; 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 245mm (9.65") 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

This letter indicates the internal construction of the tire, which you’ll generally find in two different varieties:

Radial Tires, marked with an “R,” are the most popular type, representing about 98% 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 32-45 degree angle. This construction method is used for high aspect-ratio applications, like off-road truck, drag racing, PowerSports, and reproduction tires.

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 lbs.). With a load index of 90, this tire can support 1,323 lbs.

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 mph. Manufacturers commonly add this designation in addition to W or Y ratings


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Submitted by A&A Webmaster: February 20, 2015

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