The numbers and letters printed on the tire sidewall provide vital information about the tyre's size and dimensions, and what the tire was designed for. In a tire marked "245/40R19 96Y," the "96Y" represents the tyre's load index and speed rating.
The "96" represents the tyre's load index. The load index basically shows how much weight a tire can support. A load index of 96 gives the information that the tire can handle a load of 710kg. Passenger cars and light trucks normally have tires with a load index between 70 and 110.
The "Y" at the end of the tire information tells the speed rating, or the tyre's maximum speed capability. These ratings are initially calculated in kilometres per hour, then converted to miles per hour. A "Y" goes on a tire with top speed capabilities up to 300 kilometres per hour, or 186 miles per hour. These tires are not designed for autos like a family sedan, but for a vehicle with very high speed capabilities, such as an exotic, high-performance car.
The load index and speed rating are for tires that are working adequately. The load capacity designation is for a tire that is inflated at its maximum air pressure. According to information on the Tire Rack website, the speed rating only applies to tires that "have not been damaged, altered, underinflated or overloaded."