Speed Ratings and Load Index
Speed Ratings and Load Index
After the tire size on every sidewall, you will find a number and a letter — something like 95H or 102T. These are not decoration. They tell you the maximum weight each tire can carry and the maximum speed the tire is designed to sustain safely. Installing tires with the wrong rating can create a dangerous situation that the driver never sees coming until something fails.
Load index — the number
The load index is a number that corresponds to a weight capacity from a standardized chart. For example, a load index of 91 means each tire can carry 1,356 pounds. A load index of 95 means 1,521 pounds per tire. A load index of 105 means 2,039 pounds per tire. Multiply by four and that is the total weight the tires can support. The vehicle manufacturer specifies a minimum load index for the vehicle. Never install a tire with a lower load index than specified. On a pickup truck or SUV that carries heavy loads or tows, the load index is especially critical. An underrated tire carrying more weight than it was designed for generates excessive heat, which breaks down the internal structure and leads to a blowout.
Speed rating — the letter
The speed rating letter tells you the maximum sustained speed the tire is designed for. Common ratings: S is 112 mph, T is 118 mph, H is 130 mph, V is 149 mph, W is 168 mph, Y is 186 mph. The rating is not about how fast you drive — it is about the tire's internal construction and heat dissipation capability at speed. A higher speed rating means the tire is built with materials and construction that handle the heat and centrifugal forces generated at higher speeds. Downgrading the speed rating means the tire may not handle sustained highway driving as well, especially in hot weather or under load.
What happens with the wrong rating
Installing a tire with too low a load index on a vehicle that carries heavy loads results in excessive sidewall flex, heat buildup, and potential blowout. Installing a tire with too low a speed rating on a vehicle that regularly drives at highway speeds risks tire failure from heat and structural fatigue. The tire may look fine and hold air perfectly, but the internal construction is being stressed beyond its design capability. Always match or exceed the original equipment load index and speed rating. The information is on the door placard and in the owner manual.
Never downgrade load index or speed rating below the vehicle manufacturer specification. A tire that looks fine on the outside can be failing internally from overload or heat stress. Tire failure at highway speed is catastrophic — the driver loses control instantly.