Cars and Tires: How to Buy the Best Car tires for You – Tire Size information

When you are buying tires, you want to get the best car tires that you can put on your vehicle. Motorists depend on their cars and tires both to get them to where they need to go, but it is about more than that. Your car is your freedom, and when it is not able to get you to your job daily, or out to have fun with friends, your life suffers.

Cars and tires both have similarities. The best of each are expensive, but are also an investment that you make in getting where you need to go. But buying tires does not need to be difficult, or take a long time, particularly if you have the correct information before you go to the dealership.

You should, in most cases, purchase tires that are the same size as what came on your vehicle. This allows the vehicle systems to operate as they were designed, and also ensures that the speedometer and odometer of the vehicle are as accurate as possible. In addition, in some states it is required that tires be the overall same diameter as what came on the vehicle in order to pass state vehicle safety inspections. While tire sizes may look like a strange jumble of letters and numbers, they are easy to understand.

Most modern car tires are sized with a P-metric sizing. The size of the tire is stamped on the sidewall, and will say something like P245-75-R16. This size designation tells a great deal about the size and type of tire. The “P” in the tire size stands for passenger car. These tires are usually fine for use on a passenger car, or a light half-ton truck or smaller. The designation “LT” may also be present, which stands for light truck. LT tires are generally standard on a three-quarter ton or larger truck. LT tires come in different load ranges, such as D or E. These ranges may also be referred to by the number of plies, with more plies meaning that the tire can carry a heavier load. A load range D tire has 8 plies, and a load range E tire has 10.

Some tires also carry a designation of “ST.” These are tires specifically made for trailers, and are designed for the increased stress that a tire undergoes in this type of use. ST tires are generally only approved for speeds of up to 65 MPH.

The three digit number that comes next refers to a measure of sizing. It indicates the measurement of the tire from the inside bead to the outside bead. The bead of a tire is the thick rubber component that sits against the wheel rim. With standard P-metric tires, the number refers to the number of millimeters from bead to bead, or with our example tire, 245 millimeters, or 9.65 inches.

The next number in the tire size is called an aspect ratio. The aspect ratio measures the relationship between the width of the tread of the tire, and the height of the sidewall as measured from the bead to the surface of the tread. Standard P-metric tire sizes express the aspect ratio as a percentage. With our example, the aspect ratio is 75 percent, meaning that our tire is 75 percent as tall as it is wide. Generally speaking, a tire with a lower aspect ratio number is wider than one with a higher number, and lower aspect ratio tires also have a lower profile, or a shorter length from the bead to the tread. It is common to see lower profile tires with lower aspect ratios on higher performance vehicles. This is because lower profile tires have a shorter sidewall, and are less likely to flex in high speed maneuvers.

The next part of the tire size has a combination use. Many passenger car and light truck tires will have an “R” in this position, which means that it is a radial tire. If the tire has a specific speed rating, it is shown as a prefix to the radial designation. For example, a tire labeled SR is a rated for up to 112 miles per hour. Here is a table of various speed ratings:

- M: 81mph or 130 km/h

- N: 87 mph or 140 km/h

- P: 93 mph or 150 km/h

- Q: 99 mph or 160 km/h

- R: 106 mph or 170 km/h

- S: 112 mph or 180 km/h

- T: 118 mph or 190 km/h

- U: 124 mph or 200 km/h

- H: 130 mph or 210 km/h

- V: 149 mph or 240 km/h

- Z: in excess of 149 mph or 240 km/h plus

- W: 168 mph or 270 km/h

- Y: 186 mph or 300 km/h

If you drive a high performance or sports car, it probably came with speed-rated tires from the manufacturer. Consider using tires with at least the same speed rating as what came with you car, to maintain your vehicle's original performance specifications.

The final part of the tire sizing information pertains to the wheel diameter, and is indicated in inches. The example tire is mounted on a 16 inch wheel. Passenger vehicle tires come with wheel sizes from 12 inches up to 22 inches and higher, with the most common sizes between 15 and 17 inches. The trend has been towards larger size wheels and low-profile tires. While this does enhance the vehicle's handling, the lower-profile sidewall will usually give a harsher ride. Road hazards like potholes are more likely to cause damage to wheels with low profile tires as well.

Tire sizing information is a detailed description of many aspects of the tire. A tire dealer can generally tell which tires came on your vehicle by the VIN of the vehicle, or the make, year, and model, but it can be easier when you are shopping for tires to know the size of what is on your vehicle, and provide this information when asking for a price quote.