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What size tire?

It is crucial to determine what size tire a vehicle needs to ensure proper performance.

Tire size information can usually be found on the sidewalls. [©Jupiter Images, 2009]
©Jupiter Images, 2009
Tire size information can usually be found on the sidewalls.

What size tire?

It is important to know what size tire to put on a vehicle, because every driver will need to replace his or her car's tires a few times during the life of the car. While all tires look similar, the dimensions, weight capacity, tread and types of tires are all different. Knowing what size tire a car needs also helps when dealing with tire dealerships. Even though a person works under the title of tire specialist, he or she still might not know everything about cars and tires. A little tire education benefits a driver when dealing with technicians and car professionals.

How to Determine Tire Size

ConsumerReports.org recommends looking in the doorjamb, glove compartment, owner's manual or the fuel-filler door to find the manufacturer's recommended tire size. A driver will see some numbers and letters that follow this format: P205/55R16 94V. To shop for new tires, a driver needs only to know the tires size, but understanding exactly what these numbers mean is beneficial.

The first component of the tire's measurement is the letter that precedes the numbers. Tire Rack.com gives an in-depth explanation of the most common letters seen on tire sizes:

  • "P" designates tires used primarily for passenger cars such as minivans, cars and trucks, and it is the most common type of tire on the market
  • "T" means a tire is a spare. If a driver finds this type of tire on a recently purchased used car, he or she should replace the tire immediately
  • "LT" identifies light truck tires that haul cargo or pull trailers
  • "C" at the end of a tire means the tire is for large vehicles that carry heavy loads
  • "ST" means the tires are for car or boat trailers and should not be used on cars

The first letter and number (e.g., P205) represent the cross-section width of the tire. The second number (55) represents the height of the tire. This number is a technical way of noting that the height is 55 percent of the cross section width. The last number (16) represents the rim size. The letter R simply means radial tire construction and isn't an integral part of buying the correct tire size. The tire size is also located on the sidewall of the tire, in case a driver is not able to locate the manufacturers recommended tire size within the car.

Understanding Tire and Wheel Diameter

The last number that represents the rim size is important, because technicians need to mount tires on the correct-sized rims. Wheel manufacturers offer rims measured in inches, called inch rim sizes. Inch rim sizes are the most common types of rim sizes available on the market. Rims can be as small as eight inches for small machines like lawn mowers and as large as 28 inches. There are also rims with half-inch designations, and these wheels go with larger tires for larger vehicles. A technician should not use a unique wheel size with a traditional tire, even if it is only a half-inch difference. The bead pattern of the wheel changes with the half-inch size, so wheels with unique rim sizes should never go on passenger vehicles with traditional tires.

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