If you’re a seasoned driver, you may take simple things about car ownership, like proper tire pressure, for granted. People often assume this is common knowledge, but that may not always be the case.

Each tire has a unique set of numbers and letters that provide very specific information. These numbers indicate things like tire type, measurement, serial number, load capability, and tire pressure, among other things.

Tire Pressure and Why it’s Important

One important number on a tire’s sidewall indicates tire pressure. This shows the maximum allowable pounds per square inch (psi) of air pressure a tire can hold. It’s indicated as “cold” because temperature changes cause tire pressure to fluctuate. Generally, your tires lose or gain one to two psi for every 10 degrees of temperature change. For accuracy, you should check tire pressure in the morning, after a car has been at rest, because hot tires – those having just been driven or in the sun for a long time – commonly gain five to six pounds of pressure temporarily.

Even though the tire shows a maximum psi rating, they should only be filled to the pressure listed on the tire pressure sticker located inside the driver’s door jamb, or what’s listed in the owner’s manual. The maximum psi listed is for unique handling situations, such as high speeds or rolling resistance requirements. For standard vehicle driving conditions, tires should be filled according to vehicle specifications.

Knowing the proper pressure for your tires is important because it affects how they contact the road. Too much air causes tires to be rigid, which makes the ride feel rough, while too little air can cause imprecise handling and may overheat tires at highway speeds.

According to the National Highway Traffic Safety Administration (NHTSA), there are approximately 11,000 tire-related crashes in the US each year. Proper inflation helps your tires wear evenly, and can help prevent accidents caused by poor handling or loss of traction.

Understanding Tire Measurements

In addition to maximum tire pressure, tires list serial numbers and tire measurements. The Department of Transportation (DOT) serial number includes the week and year a tire was manufactured, and certifies it was manufactured in compliance with US DOT standards.

The other code on your tires typically looks like this: P215/60R16 95H. Different tire brands or types will have different codes. Let’s look at the example above to break down what each number and letter indicates:

  • changing a tireP – The first letter indicates the type of tire. “P” stands for passenger, and is commonly seen in the United States. Tires with no letter often indicate a European tire. Other letters that commonly appear are LT, which stands for “light truck”.
  • 215 – This is the tire’s width, measured in millimeters, from sidewall to sidewall.
  • 60 – This is the tire’s aspect ratio, which is the ratio of height to width. The height of the sidewall is that percent of the width (60 percent of 215mm in this case). The larger the number, the taller the sidewall.
  • R – This indicates construction of the tire. “R” stands for radial belted, meaning the layers (or plies) run at 90 degrees from the direction of travel across the tire. It’s the most common construction for passenger tires. Other letters may include “B” for bias belted or “D” for diagonal bias belted.
  • 16 – This is the wheel diameter, it’s the size of the wheel from end to end that a tire is designed to fit.
  • 95 –Tires are assigned numbers for their load carrying capacity, known as the load index. The larger the number, the more it can carry. The typical load index for passenger vehicles is between 70 and 126. In this case, “95” indicates 1,521 pounds of carrying capacity.
  • H – This is the speed rating. Tires are designed to perform up to certain speeds. Each letter indicates a different maximum speed rating. For example, A1 is rated for a max speed of three miles per hour (mph), while A8 is 25 mph. The highest speed rating is ZR, which exceeds speeds of 150 mph. Here, “H” is rated for a maximum of 130 mph (For safety, however, don’t exceed the posted speed limit).

Now that you know what all the numbers mean, you can understand why choosing the right tire is important.

Improper Tire Care Could Lead to Vehicle Loss

Not having an understanding of your tires could lead to improper care, which is a potentially hazardous situation. This can be especially true as seasons change and road conditions become slick with ice, rain, or falling leaves. After all, a lot is riding on your tires.

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