You may have noticed your tires have a pretty long number on them. Yours might start with a letter, then a 3-digit number, followed by a few more numbers and letters. That string of letters and numbers is very important for your vehicle because it contains useful information on what tires will give you the performance to meet your vehicle’s original design specifications.
These numbers can be found in most owners’ manuals and usually on the driver’s side doorframe. Generally, it’s a good idea to stick with the vehicle manufacturer’s recommendations.
Let’s look at an example. One customer’s SUV has this number on her tires: P245/70R17 108T.
The first letter, P, means it’s intended for passenger vehicles. Some tires don’t have any letter at the beginning, which means it’s a metric tire. Some tires may also have LT at the beginning or end, which means “light truck”.
The next number, 245, in our example, indicates the tire’s width in millimeters from sidewall to sidewall. So, this one is 245 mm wide.
The number after that, 70 in our example, stands for the ratio of the height to its width, or the aspect ratio. In our example, the number 70 means the height is 70 percent of the tire width. High-performance road tires will have a lower aspect ratio than a tire designed for comfort or off-road use.
The next letter, R, in our example, is the type of tire it is, radial in this case. The next number, 17 in our example, is wheel diameter in inches.
Load index is the next number, 108 in our example. The maximum load this tire can carry is 2,205 pounds (1,000 kg). The higher the load index, the more weight it will take. To find that information, you need to reference the standardized Load Index table – which is another reason to seek expert guidance from your NAPA AutoCare Center. And finally, that last letter, T, is the speed rating, basically the maximum speed for which a tire is designed – 118 miles per hour (190 kph) in this case. Generally, the higher the letter, the higher the speed rating. Except for “H” – for some reason that slots in between “U” and “V”.
In some cases, you can change tire and wheel sizes, but it’s best to get expert help since mismatching tires and wheels with a vehicle can cause performance and safety issues. It requires resetting some of the electronic systems to compensate for the new tire/wheel size. So, consult your NAPA AutoCare Service Advisor when it’s time to put a new set of tires on your vehicle.