Speed and Jam wheels are usually 62mm in diameter and 42mm width. Their diameter and width provide skaters with a fast roll and a wide base for better traction when cornering at high speeds.
The hardness of the wheels can be attained by determining the wheels durameter. The durameter for each wheel is specified in the description by a number followed by the letter A. A lower number means the wheel will offer better grip in the turns, and a higher number means the wheel will offer a faster roll but will tend to have less traction. Durameters of 94A and lower represent Grip and durameters of 95A and higher represent Firm.