Join Date: Jun 2004
Location: Northern Nevada
Depends on a few variables, but I think the major one is tires and tire pressure. I got back into cycling on a mountain bike in the '80s after a 10-year post-college layoff, then started doing road rides with some co-workers. They all had fairly expensive bikes, but I was the new guy with a new family and couldn't afford one. I did many rides in the 30-70 mile range with a mountain bike on road tires before i finally got a roadie.
It's been several years, but as I recall, I rode at 12-13mph average on the mountain bike with knobbies at about 50 psi, 15-16 with 1.5-inch road tires at 70psi, and 17-18mph on a mid-range road bike on 28mm tires at about 95psi. That's on mixed terrain, rolling hills, suburban rides--just riding around, basically. When I trained hard, I could cruise at 21-22 on the roadie, but that's about all I had in me genetically.
Any conversion factor would be a guess, but if you're running offroad tires at the usual 45-50psi, I'd assume you could figure it's 20-25 percent harder than the same ride on a road bike.