I did 20+ mph on flats a $100 used mountain bike with a nonlockout front fork, knobby tires at 40psi for a good 2 hour ride with roadies once , when I thought a bunch of roadies were going to go easy at 17-18mph, but it ended up being pulled faster by some guys going 21+ on all the flats. Although this was clearly a draft-assisted effort, which is huge considering the upright position of a mtn bike.
I'd actually be fairly certain that nearly all racing road cyclists who can keep up with the club "B" ride would be able to do 20mph for an hour on a flat-bar road bike.
I gotta admit as well - I'm not a hugely experience road cyclist, but I've ridden with a wide range of guys, and pretty much all the guys I know who are in racing clubs, were able to do 20+mph on a road bike very quickly after getting on their first road bike. Like within a few weeks once they got used to their bike.
Last edited by agarose2000; 05-23-10 at 07:41 PM.