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How to decode the information on a Tire Sidewall

There is a ton of info on the sidewall of a tire, but how many actually know what all of it means? Consult this guide to help ensure you make an informed decision when purchasing a new set of tires for your vehicle.

  • choosing a tire

There is a ton of info on the sidewall of a tire, but how many out there actually know what all of it means? Consult this guide to help ensure you make an informed decision when purchasing a new set of tires for your vehicle.

decode tire

 

P– indicates that this is a passenger tire, commonly encountered and found on most cars, SUV’s and CUV’s. You will also find LT (light truck) and ST (special trailer) tires, which are designed to carry heavier loads and offer less sidewall flex. These types of tires are meant for bigger SUV’s, trucks and off-roaders that require a stronger commercial grade tire. While they can be used on a passenger vehicle the ride quality would be diminished thanks to the stiffer sidewalls and heavier construction.

185 – This quite simply is the width of the tire specified in millimetres.

75 – is the aspect ratio or the height of the sidewall as measured from the bead. Not actually millimeters, the 75 designates that the sidewall is 75% of the width of the tread (185 in this case). A simple formula 185*75% or 185*0.75 gives us a sidewall height of 138.75mm. A good rule of thumb to keep in mind; lower numbers designate a lower sidewall height, and higher numbers a higher sidewall height. Fast, sporting cars will usually use “low profile” tires to improve steering feel and handling, while a minivan for example will have a tall sidewall for a more comfortable ride.

 RELATEDBest All-Season Tires for 2015

R14 – Most passenger tires on the road today are of radial construction, it’s quite rare to find the old bias-ply of yore. Radial construction has the internal plies of the tire running perpendicular to the bead or centre-line of the tire. The number 14 designates the diameter of rim in inches that the tire should be mounted on. This particular tire must be mounted on a 14″ rim.

82 S – indicates the load and speed index of the tire. The numbers correspond to a standardized load /speed index chart. A handy one can be found HERE. The load index represents the amount of weight that the tire can safely carry at its maximum inflation pressure. The speed index, you guessed it, is the maximum safe speed a tire can travel when loaded. Refer to the link above to get these values for your particular tires.

While the info listed above will allow you to determine the size, type and speed rating of a particular tire, it’s performance characteristics can be determined by the UTQG ratings for Traction, Temperature and Treadwear.

winter tires

 

Treadwear – In our example the number is 360. Assuming a baseline tire would test at 100, the 360 treadwear indicates that this tire would last 3.6 x longer than the baseline. It’s important to note that this rating is specific to each manufacturer. A Michelin tire at a rating of 300 would not be the same as Goodyear tire with the same rating. Treadwear numbers should only be compared when considering tires from the same brand.

RELATED: Identifying Common Tire Treadwear issues 

Traction – is tested only under wet condtions to determine the rating. The letter corresponds to the friction co-efficient generated when the tire is dragged across the surface of the road. AA corresponds to a co-effiecient of 0.54 and above on asphalt. Traction grades ratings are AA, A, B, C, with AA being the highest C the lowest.  Any tire tested below Grade C is deemed unsuitable for road use.

Temperature – or Temperature resistance grades are the ablilty of a tire to dissipate heat build up. Typically higher grades indicate a faster top speed as these tires can resist the forces of heat better than one with a lower grade and will be less subject to blow-outs when running at sustained high speeds. Grades are typically A, B and C with C being the lowest grade possible. The Grade “A” in our example means that this particular tire can run at speeds over 115mph.

tires dot

 

Load and Pressure

The tire max inflation pressure and max load must be on the sidewall of every tire sold in North America. The max load in this example is 2000kg(4410lbs), and the safe max pressure is 110psi. When checking your own tire pressures, always ensure that the tire is cold(not recently driven on). The correct tire pressure for your vehicle will actually be found on the door jamb. This is what the vehicle manufacturer recommends, and will usually garner the best performance. If exceeding these recommended pressures (with a fully loaded car for example), never go above the max inflation pressure stamped on the tire sidewall. Exceeding these inflation or load values can lead to tire failure.

D.O.T.

This denotes the Department of Transportation and Transport Canada safety standards. This number decoded will tell you the manufacturer and plant the tire came from,  as well as the date of production. The last 4 numbers indicate the week and year of manufacture. In this case the tire was made on the 8th week of 2015. Determining tire age is crucial as some dealers can have new tires sit in the warehouses for years. Most rubber compounds used in tires today have a finite life, and tend to age, harden and crack with time and exposure to the elements, weakening the tire structure and increasing chances for failure. A good rule of thumb is 6 years. If the DOT indicates the tire is older than 6 years, it should not be put into service, or replaced if currently on a vehicle.

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