There is a lot more to tires than meets the eye, and Car Care Connection wants you to be an informed consumer. Therefore, we offer you this guide to what the various codes and numbers on a tire can tell you. As a minimum when shopping for new tires, be aware of:
- The correct tire size for your vehicle
- The tire codes
- DOT code
- UTQG code
Correct Tire Size
It is important to replace tires on your vehicle with the same size tire for several reasons:
- Buying smaller tires puts more pressure on them and can cause a loss of control, as well as faster wear over time.
- Your car’s speedometer and odometer are calibrated for the tire size originally used on it when new. If you change the size, it changes the distance the tire moves with each rotation and therefore will affect both the speed and distance recorded by the car.
- Automatic transmissions also are affected by tire changes since speed is normally used to determine when the transmission shifts. This can affect vehicle performance and fuel economy.
On the side of each tire is a code that could read “ P185/75R14 82 S” as an example. There will also be additional codes we will discuss below. Let’s break that all down:
Tire type — The P refers to a passenger vehicle tire. LT is used for light truck tires, ST for trailers requiring a thicker sidewall due to “scrubbing” that occurs when the trailer turns, and T is for spare or “temporary” tires.
Tread width — The 185 in the example refers to the width of the tread in millimeters.
Aspect ratio — The “75” after the slash mark refers to the aspect ratio or the ratio of the section height to width. The 75 means the section height is 75% of the section width. A lower number usually offers better steering behavior.
Construction — The “R” refers to a radial tire build where the internal cords of steel, nylon, or other materials are laid in a radial pattern around the tire. A “B” would refer to a bias-belt construction, and a “-“ would note a bias-ply construction which is normally a harsher ride.
Diameter — The next number can be one or two digits and refers to the diameter of the wheel or rim that the tire is intended to fit.
Load rating index— Following the tire diameter is the load rating or load index. This is a number that shows the maximum load designed for the tire. A table is needed to convert the number to the actual load in pounds; an example of this table can be found at https://rvtires.com/tire-load-capacity/?v=7516fd43adaa.
Speed rating — The last letter on a tire indicates its speed rating. This is the maximum speed allowed by the manufacturer for a particular tire design. A table showing the list of speed ratings can be found at http://tireguides.com/TireTips/TireDocument/1.
DOT and date codes — Following the letters “DOT” on the tire are a series of codes that refer to the manufacturer. Following that will be a four-digit code that indicates the month and year the tire was manufactured. For example, the value “3518” would indicate the tire was manufactured in the 35th week of 2018. This is important when buying new tires because you want tires manufactured recently and not kept in inventory for long periods.
UTQG codes — This stands for Uniform Tire Quality Guide codes, and there are usually references to Treadwear, Traction, and Temperature and are all established by the tire manufacturer. Treadwear is based on 100 equaling standard wear, therefore 200 is twice as good and so forth. Traction tests are given a value of AA, A, B, C from best to worst after the tires are tested for stopping on wet pavement. Temperature refers to the maximum speed that the tire can safely dissipate heat, with A = more than 115 mph, B = 100-115 mph, and C = 85-100 mph.
Car Care Connection can help you determine the best tires to correctly fit your car. We offer tire replacement services as well as a wide range of repair services for both domestic and import vehicles. We use the latest diagnostic equipment to service all vehicles and get you back on the road promptly. Visit us at 6503 Gateway Avenue in Sarasota or call 941-924-6199.