If you’re in the market for new tires, all of the variables in tire specifications and the confusing jargon you might hear from tire salesmen or “experts” might make your purchase rather stressful.

Or maybe you just want to fully understand the tires you already have, the concepts at work, the significance of all of those sidewall markings. What does all this stuff mean in regular terms?

­In this article, we will explore how tires are built and see what’s in a tire. We’ll find out what all the numbers and markings on the sidewall of a tire mean, and we’ll decipher some of that tire jargon. By the end of this article, you’ll ­understand how a tire supports your car, and you’ll know why heat can build up in your tires, especially if the pressure is low. You’ll also be able to adjust your tire pressure correctly and diagnose some common tire problems!

What All the Numbers Mean

Each section of small print on a tire’s sidewall means something:

Tire Type

The P designates that the tire is a passenger vehicle tire. Some other designations are LT for light truck, and T for temporary, or spare tires.

Tire Width

The 235 is the width of the tire in millimeters (mm), measured from sidewall to sidewall. Since this measure is affected by the width of the rim, the measurement is for the tire when it is on its intended rim size.

Aspect Ratio

This number tells you the height of the tire, from the bead to the top of the tread. This is described as a percentage of the tire width. In our example, the aspect ratio is 75, so the tire’s height is 75 percent of its width, or 176.25 mm ( .75 x 235 = 176.25 mm, or 6.94 in). The smaller the aspect ratio, the wider the tire in relation to its height.

High performance tires usually have a lower aspect ratio than other tires. This is because tires with a lower aspect ratio provide better lateral stability. When a car goes around a turn lateral forces are generated and the tire must resist these forces. Tires with a lower profile have shorter, stiffer sidewalls so they resist cornering forces better.

Tire Construction

The R designates that the tire was made using radial construction. This is the most common type of tire construction. Older tires were made using diagonal bias (D) or bias belted (B) construction. A separate note indicates how many plies make up the sidewall of the tire and the tread.

Rim Diameter

This number specifies, in inches, the wheel rim diameter the tire is designed for.

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How’d we solve this problem?? Built yet another storage building!

“What separates the S.drive from other tires is its construction, designed for a long life, low rolling resistance, good grip and traction…”

“So how good is the S.drive? Whether they were on the HKS 350Z or BMW 1 Series, the new Yokohama tire impressed, exhibiting excellent grip and traction. It performed far better than the stock O.E. tire, and was comparable to the ADVAN Neova.”

“The S.drive is an ideal aftermarket tire for performance enthusiasts on a slight budget.”

“It may well be the best and most cost-efficient way to instantly improve the performance of your Nissan 350z or Honda Civic.”

Sam Mitani, Road & Track – May 2007

“…it communicated its intentions better. The S.drive was really able to show its hand in the wet braking where it pulled up several feet short of Michelin’s best effort.”

“It’s very forgiving with plenty of feedback. Turn-in was predictable and allowed you to place the car precisely where you wanted it on the short track, and we found braking forces were high. The tires had ample grip and weren’t constantly understeering when pushed hard.”

“The S.drive appeared to be hard wearing. After a day of serious abuse, the tires on the 130i looked new. In fact, Yokohama brought out D1 drift racer Takahiro Ueno to provide a memorable drift demonstration in the T&E 350Z, and even after such punishment the tread blocks were barely scuffed.”

Greg Emmerson, Eurotuner – April 2007

Should I do this service when it’s recommended?

Yes. Absolutely.

Why do I have to do this?

Having the proper tire pressure is a crucial safety issue. Next to the brakes, the tires are the most important safety devices on your car. Incorrect tire pressure will compromise cornering, braking and stability. And in the worst-case scenario, improper tire pressure can lead to tire failure — and a serious accident. Incorrect tire pressure also will affect your comfort, fuel economy and tire life.

What happens if I don’t do this?

If tire pressure is too high, then less of the tire touches the ground. As a consequence, your car will bounce around on the road. And when your tires are bouncing instead of firmly planted on the road, traction suffers and so do your stopping distances. You’ll also feel a decrease in ride comfort. (Hint: If you notice that every tie you own has coffee on it, check your tire pressure. It may be too high.)

Most vehicles list tire pressure requirements on one of the door posts, most often the driver’s.

If tire pressure is too low, then too much of the tire’s surface area touches the ground, which increases friction between the road and the tire. As a result, not only will your tires wear prematurely, but they also could overheat. Overheating can lead to tread separation — and a nasty accident. One sign of low tire pressure is if your tires squeal when cornering.

And you need to check your tire pressure even if your tires aren’t leaking. Why? Because tire pressure also changes with the temperature outside. Tire pressure decreases by about 1 pound per square inch for every 10-degree drop in outside air temperature. So if you last had your tire pressure checked in July, when it was 80 degrees outside, and it’s now January — and the temperature is minus 20 — your tires may be underinflated by 10 pounds, which is dangerous.

How do you determine the correct tire pressure for your car’s tires?

A lot of people check the pressure listed on the tires themselves, but that’s actually the wrong place to look. The number on the tire is the maximum allowable air pressure — not the recommended pressure for that tire when used on your vehicle.

The recommended tire pressure is almost always lower than the maximum tire pressure. Check your owner’s manual to find out where to look on your vehicle to find the recommended measurement. This number usually is indicated either on the driver’s door pillar, the glove compartment door or sometimes on the gas filler door.

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