This isn't exactly crusing speed but on my 1993 GT Corrado mountain bike with a pair of Kenda Kross tires I did a 5 mile TT with an average speed of 18.4 MPH last August. My previous year's best was 18.1 MPH on the same course with the same bike and tires. At least I'm seeing some improvement.
I have a cousin that is extrememly fit and can sustain 18 MPH for a couple of hours on his MTB with knobby tires! I don't see how he does it as he doesn't specifically train for cycling. He just trains for all around fitness.