Tires are one of the most important parts of your car. As the only part of the vehicle in contact with the road, they have to transmit all the forces the car experiences onto the road. Inevitably you’ll have to replace your tires as a car owner at some point. But where do you start? The first stop would be to check what your current tire specifications are. This guide should be a good starting point to understand what all those letters and numbers on the side of your tire are.
After the brand name, the next largest part of text would be something like this : P225/65R17
Depending on the origin of the tire, there may or may not be a letter such as P or LT in front of the set of numbers. The P would denote “Passenger Car” and would include SUVs. The LT would denote “Light Truck” and would be for more serious SUV or Off-road tires.
The three numbers tell us the cross-sectional width of the tire in millimeters. In the picture above, the tires width, measured across the widest part of the tire when mounted correctly and inflated on the rim, is 225 millimeters.
The two-digit number after the forward slash is known as the “Aspect Ratio”. The 65 indicates that the sidewall distance, from the wheel rim to the outside of the tread, is 65% of the section width. A lower aspect ratio means a lower-profile tire with a shorter sidewall, while a tire with a higher aspect ratio will have a taller sidewall and look thicker. You can calculate it as follows:
Aspect Ratio = Distance Between Rim and Tread/ Tire Width
The R after the aspect ratio actually stands for the construction of the tire, in this case radial. Most tires made today are of radial construction and you may rarely see something such as D which would be a bias-ply construction.
Load Rating The two (or sometimes three) digit number following the main set of numbers is the load rating of the tire. This is the weight the tire is designed to be able to withstand under normal usage. If you took the load rating of your current tires and multiplied them together you will get a weight that is much heavier than your car, even fully loaded. This is done on purpose to give a safety margin.
The letter after the load rating is the speed rating. This is the maximum speed the tires can be run at without dangerous degradation of the tire due to heat. The speed rating would be important if you travelled regularly at high speeds, like on the German Autobahn. Note that the Z rating doesn’t give a maximum rating, just that it is rated to over 240 km/h.
If you look closely in-between the treads you may see some small nubs like the picture below. These are tread wear indicators and when these start becoming exposed, that tells you it is time to change the tire.
Words & Images : Kanchana Gunasekera