Determining what size tires you need is one of the most important, and least understood, aspects of caring for your car. Checking a car's tires, and possibly buying new car tires, should be a regular auto maintenance task for every car owner. The first question every tire salesman will ask is, €What size do you need?€ The correct tire for any vehicle is a unique combination of variables such as the tire width, height, diameter, load capacity rating and speed ability. The owner's manual should contain the vehicle requires what size tires the car needs and the tires' speed/load rating. If this information is not listed in the car's manual, try checking the inside of the driver's door, the glove compartment door, the fuel tank hatch and the sidewall on the car's existing tires. NOTE: All tires have their size printed on them, so it does not matter which tire is chosen to get the tire code.
The numbers to look for, to determine a car's tire size, inside your car. Once you locate the information, it should look like this: 1-2 letters, followed by 3 numbers, followed by 2 numbers, followed by an "R" and 2 more numbers. Write all of these down and bring them to an auto body shop to see what tires are available in this size.
Most tire sizes begin with a letter or letters to identify the vehicle and/or type of service for which the tires are made to be used. Below are the meanings of the most common letters. The first three numbers of the group are the width of the tire in millimeters. The second two numbers in the group are the size of the €sidewall aspect ratio€ of the tire in millimeters. (The sidewall runs from the top of the tire tread to the top of the rim.) The numbers following the "R" in the group are the diameter of the rim of the tire in inches. In the photo, the tire size is "185/75/14" tires with a "82S" speed and load rating.
Car Tire Service Type:
P = The "P" in the beginning of the size code means it is a "P-metric" size and is designed for €passenger€ vehicles such as cars, minivans, sport utility vehicles and light duty pickup trucks. If there isn't a letter before the three numbers of a tire size, then the tire is a "Metric" or "Euro-metric" size.
T = A €T€ in the beginning of the size means the tire is a "Temporary Spare" (AKA "space saver" or "mini spare") designed to be used ONLY until the flat tire on a car can be repaired or replaced.
LT = An €LT€ in the beginning of the size means the tire is a "Light Truck-metric" size used for vehicles that carry heavy cargo or tow large trailers.
LT = An €LT€ at the end of the size means the tire is either an earlier "Numeric", "Wide Base" or "Flotation" Light Truck used for vehicles that carry heavy cargo and tow trailers (Numeric sizes), use 16.5€ diameter rims (Wide Base sizes) or are over-sized tires to help the vehicle drive on top of dirt or sand (Flotation sizes).
C = A €C€ in the beginning of the size means the tire is a "Commercial" size for vans or delivery trucks that carry heavy loads. These tires also feature a Service Description and "Load Range."
ST = An €ST€ in the beginning of the size means the tire is a "Special Trailer Service" size for boat, car or utility trailers.
The rest of the numbers to look for are on the €sidewall€ of a car tire to determine the correct car tire size.
Aspect ratio: A car tire's €aspect ratio€ simply states the tire's height. The higher the number, the taller the tire's sidewall (from rim to tread) is going to be. To calculate this number, divide the height of the tire by it's width, then convert to a percentage. IE. A 50 indicates that this tire sidewall height is 50% of its section width. An average, all-season car tire will have an aspect ratio between 65 and 80.
Internal Construction: The letter (R in this case) after the 2 digit number aspect ratio states the tire's internal construction. IE. The €R€ in the P225/50R16 91S size means the tire has a €Radial€ construction so that the tire's body plies "radiate" out from the center of the wheel. Radial tires are by far the most popular type of tire today representing over 98% of all tires sold. A €D€ would mean the internal tire body plies crisscross on a €Diagonal€ and that the tire has a "bias ply" construction. Tires using this construction are for light truck and spare tire applications. A €B€ would mean the tire body plies not only crisscross the tire on a diagonal as before, but that they are reinforced with belts under the tread area. This type of construction is called "Belted,€ and is rarely used anymore.
Self-Supporting Run Flat: €Self-supporting run-flat€ tires may be identified with an €F€ right aftre the traditional letter identifying their construction (€R€ for radial).
Diameter: The tire and wheel diameter are designed to be matched together. (IE number €16€ in the photo above) Tires with their rim diameter listed in inches are called "inch rim" sizes, and most feature these.
Two MUCH less common types of tire and wheel diameters are those listed in "half" inches and those listed millimeters, which are called €millimetric€ sizes. NOTE: Tires and wheels with special rim diameters should NEVER be combined with traditional "inch rim" tires and wheels. ALWAYS confirm that the tire and wheel diameters are match before the tire is mounted on the wheel.
Load index: A car tire's €load index€ indicates the weight the tire was designed to safely support. The load index chart below matches the rating with the number of pounds the tire can support at it's maximum air pressure. IE. If a car tire's load rating is 81, it was designed to carry 1,019 pounds when fully inflated. A car's carrying capacity is determined by multiplying this weight by four (1 x 4 tires).
Speed rating: A car tire's €speed rating€ indicates how well the tire will reach and maintain a certain speed. (This number is issued by the U.S. government.) Speed ratings are the letters that correspond to the top speed. NOTE: Speed ratings tell how well the tire will handle. They are NOT recommended driving speeds! IE A higher speed rating provides better traction. HINT: Always buy 4 tires with identical speed ratings, as mixing different ones together may result in unsafe driving conditions. Don't downgrade the speed rating on them either. Simply follow the manufacturer's guidelines to buy tires with identical ratings to the car's original stock ones. The only tires that include the speed rating in the tire size today are Z-speed rated ones. For examply, after the two numbers for the aspect ratio are the 2 letters €ZR€ to identify the tire's speed rating (Z) and internal construction (R). All other speed ratings are identified in it's Service Description.
Obviously, there are other variables which effect the type of tires a car may need, such as tread wear, traction, and temperature grades. This article is merely meant to address how to read the basic car tire code system.
The numbers to look for, to determine a car's tire size, inside your car. Once you locate the information, it should look like this: 1-2 letters, followed by 3 numbers, followed by 2 numbers, followed by an "R" and 2 more numbers. Write all of these down and bring them to an auto body shop to see what tires are available in this size.
Most tire sizes begin with a letter or letters to identify the vehicle and/or type of service for which the tires are made to be used. Below are the meanings of the most common letters. The first three numbers of the group are the width of the tire in millimeters. The second two numbers in the group are the size of the €sidewall aspect ratio€ of the tire in millimeters. (The sidewall runs from the top of the tire tread to the top of the rim.) The numbers following the "R" in the group are the diameter of the rim of the tire in inches. In the photo, the tire size is "185/75/14" tires with a "82S" speed and load rating.
Car Tire Service Type:
P = The "P" in the beginning of the size code means it is a "P-metric" size and is designed for €passenger€ vehicles such as cars, minivans, sport utility vehicles and light duty pickup trucks. If there isn't a letter before the three numbers of a tire size, then the tire is a "Metric" or "Euro-metric" size.
T = A €T€ in the beginning of the size means the tire is a "Temporary Spare" (AKA "space saver" or "mini spare") designed to be used ONLY until the flat tire on a car can be repaired or replaced.
LT = An €LT€ in the beginning of the size means the tire is a "Light Truck-metric" size used for vehicles that carry heavy cargo or tow large trailers.
LT = An €LT€ at the end of the size means the tire is either an earlier "Numeric", "Wide Base" or "Flotation" Light Truck used for vehicles that carry heavy cargo and tow trailers (Numeric sizes), use 16.5€ diameter rims (Wide Base sizes) or are over-sized tires to help the vehicle drive on top of dirt or sand (Flotation sizes).
C = A €C€ in the beginning of the size means the tire is a "Commercial" size for vans or delivery trucks that carry heavy loads. These tires also feature a Service Description and "Load Range."
ST = An €ST€ in the beginning of the size means the tire is a "Special Trailer Service" size for boat, car or utility trailers.
The rest of the numbers to look for are on the €sidewall€ of a car tire to determine the correct car tire size.
Aspect ratio: A car tire's €aspect ratio€ simply states the tire's height. The higher the number, the taller the tire's sidewall (from rim to tread) is going to be. To calculate this number, divide the height of the tire by it's width, then convert to a percentage. IE. A 50 indicates that this tire sidewall height is 50% of its section width. An average, all-season car tire will have an aspect ratio between 65 and 80.
Internal Construction: The letter (R in this case) after the 2 digit number aspect ratio states the tire's internal construction. IE. The €R€ in the P225/50R16 91S size means the tire has a €Radial€ construction so that the tire's body plies "radiate" out from the center of the wheel. Radial tires are by far the most popular type of tire today representing over 98% of all tires sold. A €D€ would mean the internal tire body plies crisscross on a €Diagonal€ and that the tire has a "bias ply" construction. Tires using this construction are for light truck and spare tire applications. A €B€ would mean the tire body plies not only crisscross the tire on a diagonal as before, but that they are reinforced with belts under the tread area. This type of construction is called "Belted,€ and is rarely used anymore.
Self-Supporting Run Flat: €Self-supporting run-flat€ tires may be identified with an €F€ right aftre the traditional letter identifying their construction (€R€ for radial).
Diameter: The tire and wheel diameter are designed to be matched together. (IE number €16€ in the photo above) Tires with their rim diameter listed in inches are called "inch rim" sizes, and most feature these.
Two MUCH less common types of tire and wheel diameters are those listed in "half" inches and those listed millimeters, which are called €millimetric€ sizes. NOTE: Tires and wheels with special rim diameters should NEVER be combined with traditional "inch rim" tires and wheels. ALWAYS confirm that the tire and wheel diameters are match before the tire is mounted on the wheel.
Load index: A car tire's €load index€ indicates the weight the tire was designed to safely support. The load index chart below matches the rating with the number of pounds the tire can support at it's maximum air pressure. IE. If a car tire's load rating is 81, it was designed to carry 1,019 pounds when fully inflated. A car's carrying capacity is determined by multiplying this weight by four (1 x 4 tires).
Speed rating: A car tire's €speed rating€ indicates how well the tire will reach and maintain a certain speed. (This number is issued by the U.S. government.) Speed ratings are the letters that correspond to the top speed. NOTE: Speed ratings tell how well the tire will handle. They are NOT recommended driving speeds! IE A higher speed rating provides better traction. HINT: Always buy 4 tires with identical speed ratings, as mixing different ones together may result in unsafe driving conditions. Don't downgrade the speed rating on them either. Simply follow the manufacturer's guidelines to buy tires with identical ratings to the car's original stock ones. The only tires that include the speed rating in the tire size today are Z-speed rated ones. For examply, after the two numbers for the aspect ratio are the 2 letters €ZR€ to identify the tire's speed rating (Z) and internal construction (R). All other speed ratings are identified in it's Service Description.
Obviously, there are other variables which effect the type of tires a car may need, such as tread wear, traction, and temperature grades. This article is merely meant to address how to read the basic car tire code system.
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