Understanding all the writing on the sidewall of a tire can be somewhat overwhelming at first glance. It helps to know what characters you are searching for in order to obtain the
tire information you desire. For example, if the tire code was 225/45R17 75L, then 75 would signify the load index and L would signify the speed rating of the tire.
The load index tells you how many pounds a tire can safely carry. As you can see on the load index chart below, the 75 in the example above has been assigned a load carrying capacity of 853 lbs per tire. Multiply the load carrying capacity by the number of the tires on the trailer. As long as that sum is greater than or equal to the assigned GVWR of the trailer, then the tire’s load index is sufficient for its application.
Speed ratings are based on laboratory tests and were established to match the speed capability of tires. In the same tire code example stated previously, L would signify the tire speed rating. According to the attached chart, the speed rating for L is up to 75 mph. It has been proven under testing conditions, that tire can operate for a certain period of time up to 75 mph.
Motor vehicles should always be operated according to the speed limit of the roads they travel on. Never exceed the speed rating of the tow vehicle tires or the trailer tires; drive according to whichever rating is lower. Trailer tires commonly have tire speed ratings in the 65 – 75 mph range. Unless otherwise noted, the speed rating for trailer tires is 65mph.