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Tire speed ratings first originated in Europe in the 1960s, as a way to make sure increasingly faster vehicles were equipped with appropriate safe tires. These early speed ratings set the foundation for the current speed rating system used by the DOT and tire manufacturers.

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Every tire approved for highway use by the Department of Transportation comes with a particular speed rating. The speed rating is a letter corresponding to the maximum safe speed at which a tire can be driven, as determined by laboratory testing.

During laboratory testing, a technician places a properly inflated tire into a large diameter drum. The drum rotates the tire for 10-minute intervals at increasingly higher speeds. The technician verifies the tire’s speed rating by rotating it at a maximum speed, and then ensuring that the tire retained its integrity during the rotation. These tests are conducted in controlled settings, at room temperature on a smooth surface. Because speed ratings are determined in ideal conditions, they do not indicate the speed that you can consistently maintain. They instead describe the tire’s capacity to reach and sustain a certain speed for a particular length of time.

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Speed ratings can affect the way you drive. To maintain your vehicle's original speed capability, you need to equip it with tires that meet or exceed the original equipment tire's speed rating. Mixing tires of different speed ratings can reduce your maximum safe speed. If you equip your vehicle with tires of different speed ratings, the lowest speed rating will determine your vehicle's maximum speed of which the tires can safely handle. Be aware of your original equipment tire's speed rating to avoid this reduction.

Look for a tire's speed rating by checking the end of its size code. Most likely, the last figure in the tire size code will be a letter representing the speed rating. These letters correspond to the tire's maximum safe speed. Letters farther along the alphabet correspond to higher speeds, with "H" being an exception up to 130 mph). In the illustration, below, the tire has a V speed rating.

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Here are the letter ratings for tires that can be safely driven 149 mph.or faster:

V= up to 149 mph.

W= up to 168 mph.

Y= up to 186 mph.

(Y)= 186 mph.+

Z= 149 mph. and over

The "Z" rating may appear on any tire rated above 149 mph. Tires with "Z" ratings were originally thought to be to the highest rated tires a vehicle would require. However, the development of faster vehicles led to the production of two new speed ratings, "W" and "Y." These two ratings provide a better indicator of the tire's capacity for speeds higher than 149 mph. The "Z," "W," "Y" and (Y) ratings appear on tires for sports cars, muscle cars (like the Challenger), exotics, or other vehicles engineered for ultra-high speeds.
 

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Not mentioned and I cannot find it but they rating is for a speed with a certain load range. I couldn't find the load amount specs.
 
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