Let’s take an example of the tyre measurement which is 225/50R17 98H. The first combination set of letters and numbers (e.g. 225/50R17) describes the tyre size and measure, and the last combination set of numbers and letter (e.g. 98H) represents the tyre’s service description.
This is the width of the tyre, measuring from sidewall to sidewall, in millimeters. A 225 means the tyre is 225 millimeters, or 22.5 centimeters, wide.
Aspect Ratio or Tyre Profile. This is the sidewall's height (or the depth of the wall at the side of the tyre) that takes measure from the inside diameter to the outside diameter. The profile is expressed in percentage of the width of the tyre. A 225/50 means the sidewall is 50% of the tyre width, making it approximately 112.5mm tall. A low profile tyre is commonly found in the high performance vehicles or sports cars with aspect ratio of 50% to as little as 30% with ultra-low profile tyres. As aspect ratios decreases, the tyre's firmness increases. As a result, the low profile tyres are able to provide greater control and better handling on corners, they can also give a slightly harder or bumpy ride. However, as precautionary, fitting a higher profile than manufacturers’ recommended spec on the same size wheels will alter your speedometer reading and may cause you to travel faster than the actual reading as shown on the dashboard.
The tyre's type of construction or simply, Radial construction. The Radial design means the cord plies are arranged at 90 degrees to the direction of travel, or radially (from the center of the tyre). All vehicles produced after 1980s use radials.
This is the tyre's inner diameter, which matches the diameter of the wheel rims, measuring in inches.
The Load index is used to compare relative load carrying capabilities of the tyre (the load index and its relative weight load are shown in table below). A 98 load index means the tyre is certified and able to carry up to 1653 pounds (or ~750 kgs) at its maximum inflation pressure. For example, if a car weighs 1800 kgs, then each tyre handles 450 kgs with no passengers and cargo. A higher load index tyres are required if you intend to load heavy stuffs in the car boot or towing a heavy trailer. Add up all the load weights and divide it by four to find out if you have the right tyre and within the allowable weight limit, and also make sure your replacement tyres have a corresponding or higher load index.
The speed rating is always expressed as a letter, and it usually match to the vehicles’ top speed as recommended by manufacturers. It is an indication of the ability of the tyre to dissipate heat to avoid a blowout when vehicle is travelling in high speed as heat buildup faster. Generally, the tyres with higher speed ratings are constructed to able to handle heat better. Those letters and the corresponding speeds that the tyres are capable to handle are included in this chart:
Now you will never again need to ask the guy in the tyre workshop: “What do the numbers on the tyre mean?” Instead just simply refer to our little tyre guide here.