The 113/108 M that you are finding on the tire like the Karrier ST225/75R15 Radial Trailer Tire # AM10256 is referencing the load rating and speed rating of that particular tire.
The first number 113 represents a 2,535lb weight capacity per tire when used on a single tire axle.
The second number 108 represents a 2,205lb weight capacity per tire when used on a dually tire axle.
Finally the M is the speed rating of 81mph. view full answer...