A while back I asked if anyone knew what the speed difference was between bike types: road, hybrid, mountain. I didn't get a good answer so I did some testing of my own. I post the results here because it showed the MTB to be much better than I would have guessed. Had I posted this in the hybrid/road section someone might have sought me out and hunted me down in my bed.
I used an old, but sound, Cilo steel road bike circa 1986 with 23X700 slicks for a road bike.
My Kona Dew Deluxe with 30X700 slicks for a hybrid.
Mountain bike was a Kona Fire Mountain hardtail. This bike has Kenda 2.25X26 road tires with very fine tread. (no mountains in Key West)
All tires inflated to full pressure. Over a weeklong period I ran all three bikes over an 8 mile up-and-back course early in the day when traffic was very light. Not once did I have to really slow down for cars at intersections. I did not try to race. This is important. I can no longer go real hard for 8 miles because I never do it. So it was my effort to take the course at an average commuting pace. The same level of effort I normally ride. Each bike was ridden 3 times. All speeds posted are averages. Remember I am 52 years old and not related to Lance Armstrong.
Road bike average speed was 16.7 mph.
Hybrid average speed was 14.0 mph.
Mountain bike average speed was 13.3 mph.
I was very surprised to see how relatively quick the MTB was compared to the road bike and especially the hybrid. Funny thing was I felt fast on the hybrid. But it was no big impovement over the MTB. The road bike was riden from the drops. (this nearly killed me - my lower back)
You can't compare a MTB to any road bike but they give away little to hybrids with the right tires. And they do it in greater comfort with the fat tires and front suspension.