Originally Posted by MrCjolsen
After about six weeks of not riding, I bought a road bike (a trek 1000c), expecting to gain the 4-5 mph the nice man at the bike shop said I would.
After two weeks and 144 miles with the road bike I am holding about the same speed as I did on the MTB.
There's your answer. Deconditioning. It takes a lot longer to build up conditioning than it takes to lose it. I think you'll find yourself improving your average speed over time as you start riding regularly again.