Tyres are arguably the most important part of a car. They are the only parts of your car that touch the ground and as such are the final factor in how quickly your car accelerates, how well it handles and how quickly it slows to a stop. At the same time, however, they are arguably the most misunderstood and underestimated parts of a car, especially when it comes to moving away from the factory specifications. More times than I can count, I’ve seen car owners spend crazy money on wheels to then fit them with mediocre tyres, get plus-sizing all wrong, or a combination of both. This 3-part series will outline and explain tyre specifications, address common myths about tyres and explain how to choose tyres to enhance your car’s performance. Hopefully at the end of this series you will have an appreciation on how much thought should be given to the tyre you choose for your ride.
What do all those numbers and letters on my tyre mean?
The most noticeable markings are the dimensions of the tyre. This would be the 225/50 R 17 you see in the image above. You can read them as follows:
- 225 is the Section Width of the tyre. This is measured in millimeters from one sidewall to the other.
- 50 is the Aspect Ratio. This number refers to the sidewall of the tyre, and is actually a percentage of the Section Width. So in the case of the tyre above, the sidewall is 50% of 225 which equals 112.5mm.
- R designates the tyre as a Radial Tyre. Radial tyres have layers of material (plies), whose cords run perpendicular to the circumference of the tyre, as shown below. It is the industry standard for essentially all passenger vehicles today.
Radial Tire Construction
- 17 refers to the Wheel Diameter of the tyre. This measurement is in inches and specifies the diameter of the wheel the tyre should be paired with.
Next to the size marking are the load and speed ratings. Going back to our example:
- 98 is the Load Index. This is the maximum load capacity of a single tyre. The number corresponds to a weight in pounds that you can find on a Load-Carrying Capacity per Tyre chart. In this case, 98 means a load carrying capacity of 1653 pounds. This weight is multiplied by the number of tyres to get the total load capacity. It is extremely important to choose a tyre that meets or exceeds the factory specified required load index.
- H is the Speed Rating. This denotes the maximum speed a properly installed and inflated tyre can be driven on. In our example, H translates to 130mph or 210km/h. Sometimes a Z may be included in the tyre dimensions (e.g. 225/50 ZR 17). This denotes that the Speed Rating is in excess of 149mph or 240km/h. Such a tyre will either have a W (168mph or 270km/h), Y (186mph or 300km/h) or (Y) (over 300km/h) rating.
Usually close to the dimension and speed/load markings, you will find the Maximum Pressure and Load numbers, shown in the image above.
- Maximum Pressure is the maximum air pressure the tyre can hold when in operation (51 psi in the case of the above example). This is not to be confused with the Recommended Tyre Pressure, which is found in the door jamb or your car owner’s manual.
- Maximum Load is the load capacity of the tyre at maximum inflation pressure. This is not to be confused with the Recommended Load Capacity found in the owner’s manual.
Exploring your tyre further, you will find the Department of Transportation Code or DOT Code, shown above. This code indicates that the tyre has passed all minimum standards in the U.S and is used by the Department of Transportation to track tyres in the event of a recall. The example shown can be broken down as follows:
- 4B identifies the plant where the tyre was manufactured.
- 08 is the tyre size code.
- 4DHR is an optional code that refers to a brand or other specific characteristics of the tyre, such as construction, tread pattern and tyre category.
- 2910 indicates the week (29) and year (10) that the tyre was produced. It is recommended that you replace tyres that are 6 or more years old, regardless of their tread depth.
The last piece of information to look for on a tyre is the Uniform Tyre Quality Grading, or UTQG. The DOT requires all manufacturers to grade their tyres under the UTQG system to establish ratings for treadwear, traction and temperature resistance.
- Treadwear (300 in the above image) refers to the durability of the tyre. The Industry Standard reference number is 100, so 300 indicates that the tyre has a projected tread life 3 times longer than the Industry Standard. There is room for interpretation for this number, however, as manufacturers grade their tyres after only a small amount of treadwear has occurred.
- Traction (AA) refers to how quickly the tyre can stop in a straight line on wet asphalt and concrete. A tyre’s traction rating can be AA, A, B or C, with AA being the highest.
- Temperature (A) refers to the tyre’s resistance to generating heat at high speeds. A tyre’s temperature rating can be A, B or C, with A being the highest.
EU Tyre Labels
If your new tyres have been imported from Europe, you may see a Tyre Label stuck to the tread as shown in the image above. Since November 2012, all tyre manufacturers are required to specify fuel consumption, wet traction and noise level for every tyre sold in the EU. These characteristics are measured as follows:
- The energy efficiency of a tyre is measured by its Rolling Resistance. Reduced rolling resistance lowers fuel consumption and CO2 emissions. Rolling resistance ratings range from A to G, with a 7.5% increase in fuel consumption between A and G.
- Wet Grip is classified by the stopping distance from 80 km/h on a wet surface in a straight line, similar to the UTQG traction rating. Ratings range from A to G, with a 18 meter difference in stopping distance between A and G. D and G are not used currently.
- Noise is measured in decibels and has 3 classes, represented by the sound waves on the label.
This brings us to the end of part one. Hopefully you now have a better understanding of the information that can be found on your tyre. Be sure to lookout for Part 2, where we will debunk some of the most common myths about tyres. Some may shock you!