For most people a tire size specifications is just a mixed-up garble of letters and numbers that only increase the price of the tire the larger they become. Believe it or not, this is not just a ploy developed by tire manufacturers to confuse vehicle owners into spending more money. Every number and letter has a meaning. Understanding these specs will help you identify the best tires for your vehicle in both size and performance. Here’s tire sizes explained.
Let’s start right at the beginning. Most tire sizes will begin with the letter P. This is simply stating that the tire is a passenger car tire. The term P-metric is the U.S. version of the metric sizing system. The other letters you may see in place of the P at the beginning of the size chart are LT. This designation is stating that the tire is meant for use on light trucks. It’s never wise to use an LT tire on a passenger car or vice versa.
The next number in the size spec is a three digit number stating the number of millimeters the tire is across. For example, in the tire size P235/75R15, the number we’re talking about is 235. That’s the distance of measurement from sidewall to sidewall, 235mm. This number is important to know since the bigger it is, the more tire you have making contact with the road.
The next number, in this case it would be the 75, is the aspect ratio. This is the size of the sidewall of the tire, or the area from the rim to the outer edge of the tire. This number states a percentage of the overall width. For instance, the 75 signifies that the sidewall height of the P235/75R15 tire is 75% of 235mm, or roughly 176mm. The smaller this number is, the stiffer the tire is going to feel since it won’t have as much give in the sidewall. Smaller numbers is this slot, such as 30 and 40, are considered to be what is called a low-profile tire. These tires typically provide more feedback and crisper handling.
Next comes another letter. Most commonly, this letter will be an R. This is the tire construction designation. It states how the tire plies are constructed inside the tire carcass. The R simply means that this is a Radial tire. Other designations you may see are B, meaning the tire is of a belted bias construction, or D, meaning a diagonal bias construction.
The final number in the tire’s size is the simplest. It is the overall diameter of the tire’s rim. In layman’s terms, it’s the size of the hole in the middle of the tire. It is measured in inches; a value of 15 in this position means the tire is using a 15” rim.
After the tire’s size you’ll probably notice a couple other letters and numbers. These numbers are important for knowing how you want to utilize your tires. They are the tire’s speed and load ratings. They let you know what the tire can handle speed-wise and what they can carry in terms of load weight. These letters and numbers are designations of those specs. For example, you may see a tire that says 89H right after its size. The first part of this spec is the number 89. This is the tire’s load index. This tire has an industry standard maximum load of 1,279 lbs. Knowing your load index will help you decide which tire better fits your needs should you be doing any heavy hauling. A load index chart is available on most tire manufacturers websites.
The letter immediately after the load index is the speed rating. Like the load index, it designates what the maximum service speed for that particular tire is by using a coded letter to coincide with that speed. For example, the letter H signifies that tire is rated to speeds of 130 mph. Tires using the older European sizing system will list the speed rating within the size of the tire. For example P215/65HR15. A list of speed ratings and their accompanying letter designation can also be found on many tire manufacturers websites.
Understanding these specifications will help you pick the best tires for your vehicle. There are just a couple of important things to remember when selecting your tires. First, never change the size of the tire your vehicle calls for. There are different sized tires which may be comparable for your vehicle should you wish to switch to a lower-profile tire, but always make sure you check with your owner’s manual or tire dealer to see which sizes are compliant. You can also usually find your tire size on the door placard just inside your driver’s side door. Second, never mix and match tires. Having four different brand tires on your vehicle will result in poorer handling and grip on the road. Each tread pattern has its own type of performance. As such, going with different patterns all around your vehicle will cause the vehicle to have less handling and a higher risk of spinout. Know your tires and you’ll have a much better knowledge of your car.