I can't answer your question 'cause I'm on a mtb too. But I'm riding it on the road more and more. On a 20 mile loop with about 1,000 ft of ascent, the computer says I average 16 mph, including stop signs.

It's a squishy 29er. I'm looking at getting a road bike and hope it will be good for an extra couple mph, but we'll see. Anybody have experience going from a decent mtb (30 lbs) to a good road bike?