Rubber and Ratings: How Tires and Top Speeds Are Related

January 26, 2010
Last fall, we joined Continental Tires at the Nardo proving ground in Italy, where eighteen tuner cars were invited to show off their top speeds on the 7.8-mile circular track. With our archaic speed limits in the United States, we often forget that you need serious tires to survive serious speeds. And serious speeds were definitely part of this program-have a look.
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You may never achieve speeds like this at home, but all new car and truck tires sold in the United States have speed ratings on them. A tire's ability to deal with high speeds doesn't necessarily increase its handling and braking performance, but generally there is a strong correlation-so if you're looking for a higher-performance tire, look for a higher speed rating.
In earning the DOT's approval for U.S. sale, a tire needs to be tested only to a top speed just shy of 100 mph. European regulations call for ratings to be verified by running the tire up to the claimed speed in increments of 10 kph for ten to twenty minutes at each step. Although the highest current marking allows manufacturers to claim a tire that's capable of speeds only "above 186 mph," companies sometimes publish a more precise number for how fast their tires can safely go. Michelin, which makes original-equipment rubber for the Bugatti Veyron, certifies those tires to the car's 253-mph top speed. Continental makes an entire line of high-speed tires, called the Vmax, which have either 360-kph (224-mph) or 400-kph (249-mph) ratings.
Speed ratings are found on the sidewall of a tire, in what's known as the service description. The letter following the two- or three-digit numerical load index indicates the highest speed at which a tire can safely be driven. The "Y" service description surrounded by parentheses indicates that the tire can be driven in excess of that speed rating. All speeds are converted from kilometers per hour, hence the seemingly odd break points. The old "Z" rating (more than 149 mph) has been largely replaced by the "W" and "Y" designations. Some common ratings include:

S: 112 mph
T: 118 mph
H: 130 mph
V: 149 mph
W: 168 mph
Y: 186 mph
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Check out our February 2010 issue for information on peak speeds reached during NARDO event for each of the eighteen tuner cars.


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