Tires are rated according to the amount of speed they can safely hold traction to and load they can carry. The most commonly used tires for family vehicles are rated between 112 mph and 118 mph with a maximum load index rated at under 1,300 pounds.
The speed rating of the tire is marked by a letter. A higher letter indicates a higher top speed that the tire can hold traction to. For example, L-rated tires manage to hold traction up to 75 mph and V-rated tires can exceed 145 mph. The speed rating only reflects the maximum speed allowed on a healthy tire. Older tires that have a dry-rotted surface or a leak can loose traction or blow-out. Tires with recent repairs should also be kept under the speed rating of the tire to avoid any additional damage.
The load index is the number placed before the speed rating. A load index refers to the amount of weight a single tire can hold. These numbers normally range between 70 and 110, or 751 pounds and 2,337 pounds. When the tires are all used in unison, they are able to safely support the weight of the chassis, along with the passengers and their belongings.