As noted by others, MTB's typically have a wider range of gears on the rear cluster, reflecting the fact that off-road terrain varies more than roads.
Top speed on an MTB is typically limited by the big ring on the front. On a road bike, the big ring will have 52-55 teeth, on an MTB, only 42 or so. Spinning at maximum RPM (assuming you're strong enough), the road bike has a 20% advantage in distance traveled per pedal stroke. However, even if you swapped to a bigger ring on your MTB, you still have to overcome the additional weight, aerodynamic drag, and rolling resistance.
