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What Do the Numbers on Tires Mean?

Dec 31, 2018

There are a lot of numbers on the side of a tire, but what do they all mean? You can typically separate them out based on the other letters or numbers they are located next to. There are a myriad of them, but let’s start with the max pressure, which is a number followed by PSI.

 Meaning "pounds per square inch," this is usually a maximum air pressure rating for the tire which is not necessarily the amount of pressure that you’ll want to run in your tires. Typically, you would find the correct inflation information in your owner’s manual or on a label affixed to the door jamb. Over-inflating your tires can be just as dangerous as under-inflating them, so you don’t want to take this for granted.

The next most common reason for looking is probably related to the tire size itself. This is an important number to know if you are shopping for new tires. Looking at the side of your tire you might see something like P195/60R15 88H SL. Let’s break that down:

The ‘P’ just refers to the tire being a metric passenger car tire, which is probably the most common thing you’ll see these days. Some other designations you might see are ‘T’ for Temporary (spare) tires or ‘LT’ for Light Truck, and ‘ST’ for trailer. You want to make sure you are putting the correct tire on the correct application because a passenger tire is not designed to fit on a spare wheel and vice versa.

The first three-digit number ‘195’ refers to the section width, or the width of the tread in millimeters. This is the part that contacts the ground and not necessarily the overall tire width. Therefore, a higher number is wider, and a lower number would be narrower.

The next two-digit number in the tire size can be a little tricky to understand. In this example, ‘60’ is known as the aspect ratio and refers to the height of the tire sidewall relative to the section width. In this case, the height of the sidewall would be 60% of the section width. Thus, a wider tire would have a taller sidewall given the same aspect ratio.

‘R' stands for radial. Not many modern passenger vehicles are going to use anything other than a radial tire. But, if it were ‘B,’ it would be bias belt, or ‘D’ would refer to diagonal belt.

The number 15 refers to the wheel diameter in inches for which the tire was designed to fit. Occasionally you may see a half-size rim measurement, but this is relatively uncommon. There some even more rare circumstances where this number might be expressed metrically.

Next would be the ‘88’ which is the load index. This is as it sounds and the higher this number is, the higher the load index. This is often one if the more overlooked specifications and can be extremely important, especially when tires are used in more demanding applications. 

Finally, the ‘H’ refers to the speed rating. In this case that is 210kph/130mph. Typically, the further along you get in the alphabet, the higher the speed rating with the highest being Z-rated. You would typically only find these speed ratings on extremely high performance vehicles.

You'll want to do your research and be sure that all of these numbers are suitable for the vehicle that you're tire shopping for. After all, the tires are the only thing that will ever be in contact with the road (ideally, of course), so be sure that you choose the right tire for your vehicle.