In general, tyre speed rating refers to the maximum speed at which tyres can safely run without the danger of failure. In other words, it is the minimum riding standard that can be safely handled by the tyres. However, speed isn’t the only factor that governs this parameter. It also depends upon the load on the vehicle and the condition of the road. Speed rating is an indication of tyre’s strength and durability. Installing a proper rated tyre provides better cornering response, in turn ensuring safer handling.
Laboratory tests are carried out to determine the speed rating for specific tyre types. It is done by pressing the tyre against a large metal drum. Then the speed of the tyre is increased intermittently with an increment of 6.2 mph at each 10 minute interval. This process is continued until the required speed is achieved.
It is extremely important that the tyres of a vehicle are up to the minimum standard put by the manufacturer. It is illegal to have a lower rated tyre in the vehicle with respect to the load which may also invalidate the car insurance. However, when lower rated tyres are installed, which is common for winter tyres, it is recommended that a label to be put on the vehicle with a warning message specifying this information.
A vehicle should not run with tyres of different speed ratings. This may result in unpredictable steering. But when different rated tyres are needed to be installed, it should be done in like pairs on the same axle; while the lower rated tyres are put on the front axle. The minimum rating will be counted as the speed rating of the entire car in this case.
The speed rating is stamped on the side of a tyre. It is usually a letter at the end of a series. The most common speed rating nomenclature consists of letters from N to Y. N is the lowest speed rating which refers to speed not exceeding 140 km/h. The letter Y refers to speed over 240 km/h but not exceeding 300 km/h.
It should be remembered that these speed ratings may not hold true for adverse weather or unusual behavior of the vehicle. Also a vehicle which has been damaged, or running with over inflated or under inflated tyres will also not reflect the speed rating.