The most common tire speed ratings, speeds and vehicle usage are as follows:
We can thank Germany's famous Autobahn for tire speed ratings. Tire speed ratings range from A (the lowest) to Y (the highest). But the chart is not completely in alphabetical order. For example, H is between U and V, with the common perception that H stood for "high performance" at one time. As manufacturers continue to add speed to their vehicles, tire speed ratings evolve to match the speeds. For example, Z was the highest rated speed at 149+ until W & Y were used to match the higher speeds of exotic sports cars.
Speed ratings make a difference not only in regards to speed, but in regards to ride comfort, wear and cornering ability. Typically, the higher the speed rating, the better the grip and stopping power, but the lower the tread life. You can always increase the speed rating of the tires on your vehicle for improved performance, but can never decrease it without reducing the vehicle top speed to that of the lower speed rating selected.
If tires of different speed ratings are mounted on a vehicle, the lower speed-rated tires should be placed on the front axle regardless of which axle is driven. This is to prevent a potential oversteer condition. Vehicle handling may be affected, and the vehicle’s speed capacity is now limited to the lowest speed-rated tire. For best performance, it is recommended that the same size and type of tire be used on all four wheel positions.
235/55R17 99H- The load index (99) is the tire size's assigned numerical value used to compare relative load carrying capabilities. The higher the tire's load index number, the greater its load carrying capacity.
97 = 1,609 pounds
98 = 1,653 pounds
99 = 1,709 pounds
A tire with a higher load index than that of the Original Equipment tire indicates an increase in load capacity. A tire with a load index equal to that of the Original Equipment tire indicates an equivalent load capacity. A tire with a lower load index than the Original Equipment tire indicates the tire does not equal the load capacity of the original and should not be considered for installation on the vehicle.
Typically, the load indexes of the tires used on passenger cars and light trucks range from 70 to 130.