Geez, I wish I could maintain 8mph on my mountain bike. If you are riding trail and if there is climbing involved, that's a good average.

On flat roads, I can maintain an average of 18 mph so I'm no slouch but on trail there is no way to roll that fast. My road tires are 23cm and my MTB tires big ole 2.2's - huge difference! Bike geometry and weight also are factors. My MTB fully just weighs more than my road bike (21#s v. 28#s).

If speed is what you want on trail, then work at it. Push each ride, do intervals. Each time you will get alittle faster!