Tire load index is the numerical portion of the tire service description imprinted on the sidewall of a tire. The overall tire service description consists of a two or three-digit number followed by an alphabet and can be found immediately after the tire size. The alphabet portion of the tire description represents the speed rating which is a required element (according to US Department of Transportation) of a tire’s service description since 1991 for all except z-speed-rated passenger tires. Whereas, the numerical portion corresponds to the load index rating for a tire.
Load index ratings range from 71 to 125. The number is not the load capacity of the tire; it must be correlated using a chart like the one below to determine the maximum load rating. Load index ratings are based on the maximum allowable fill pressure of the tire as listed on the sidewall.
Let’s consider the following tire size as an example, 94 is the load index rating, and is the speed rating. Based on the chart below, the corresponds to a load index rating of 1477 lbs at maximum tire pressure, so the load-carrying capacity of all four tires would be 4 x 1477 lbs = 5908 lbs.
It is important to note that the maximum load capacity of the tires may not be the actual capacity, since the air pressure in the tires will probably be set by the manufacturer at a level below the maximum air pressure listed on a tire’s sidewall. Also, the total vehicle weight must be factored into the equation when determining the optimal tire from a load-rating perspective.
For safety reasons, industry recommendation is that a driver should not replace a lower load index rating than the OEM tires specified by the manufacturer. In fact, due to liability concerns, most installers will not sell nor install a lower load rated tire than OEM recommendation.