The durometer number only indicates the hardness of the urethane and doesn't give any insight as to how fast or how low the rolling resistance might be for that wheel. While the durometer number can be useful for fine tuning some performance aspects, it doesn't matter when determining if a wheel will fit into your skates or not. Inline wheels with durometers from 74a (very soft) to (90a very hard) can be found. However for recreational and fitness skating, inline wheels in the 82a to 86a hardness range are best. The 84a durometer is the most popular and typically offers the best mix of performance, long wear and a reasonably smooth ride for most people. While it's generally true that a harder wheel will last longer remember that softer wheels will ride more smoothly on a rough trail and a harder wheel will be noticeably more harsh on the same trail.