Tire speed ratings, demystified

If you’re going this fast, you’d better be on the Autobahn.

Did you know your tires are rated for their maximum speed? A careful inspection of the various letters and numbers on a tire’s sidewall will tell you nearly everything you need to know about it, including its size, construction, and speed rating, among other details.

The speed rating is assigned by the federal government – it’s a one-letter designation found after the tire size, along with a two-digit load rating. This rating only certifies a tire’s capacity to endure high speeds; a side benefit is that the higher ratings typically ensure that the tire offers superior ride and handling.

Tire speed ratings can be pretty confusing. You’d think that “Z” would be the highest rating, but it isn’t – “Y” is. To make sense of everything, we’ll need to examine the history of these ratings.

Born in Europe

side wall

Tire speed ratings originated in Europe in the 1960s, according to Tire Review. Originally only foreign-made tires offered ratings, but the United States eventually jumped in and established ratings per Federal Motor Vehicle Safety Standard (FMVSS) 109.

European officials automatically assigned a “Z” rating to tires operating at speeds at 149 mph and above. The last letter of the alphabet was chosen because at the time there wasn’t a car built that would go much above 149 mph. And with no speed limits on Germany’s autobahn, the “Z” designation seemed sufficient.

But as cars have evolved to reach ever higher speed levels, the “Z” rating was no longer sufficient.

The U.S. government steps in

So, the federal government jumped in and created a revised speed rating criteria, beginning with “B” and ending with “Y”. You won’t find “B” tires on passenger vehicles, as they’re rated to only 31 mph. But you might find an “M” or “N” tire, rated to 81 and 87 mph, respectively. Those two ratings are usually assigned to temporary spare tires – you know, the donut tires found in many late-model car trunks.

After “N” you’ll find “Q” rated tires; for some reason “O” and “P” are not used. Mysteriously, the “H” rating was also not used (along with “A” and “I”). But the “H” rating (130 mph limit) was later added, filling a gap between the 124 mph speed rating for the “U” and the 149 maximum rating for the “V.” However, “H” and “W” (189 mph limit) tires once previously reserved for performance cars are now outfitted on some family cars like Honda Accords (please don’t try to drive your Accord 189 mph). Such tires help manufacturers meet the latest safety standards, and also provide improved acceleration, braking, and handling. The downside is that these tires may wear out faster.

Your next tires

If you need new tires, what’s the best approach? That’s easy – choose the same tires that came with your vehicle. If you want to make changes, going up on the speed rating may improve cornering. On the other hand, if you downgrade your speed rating the result may be poor handling and unstable steering – not something we recommend.

Want more information? Read our article on speed rating and load index.

 

 

Photo by: Michael Pereckas

 

Leave a Reply

Your email address will not be published. Required fields are marked *