Unfortunately, this is probably a significant overestimate, especially given that he has slicks on. On a related thread, several people cited 12% as a differential or a little higher. this implies about 2 mph. I can average close to 20 on the road on a MTB with big knobbies and front shocks, and can do 18+ on the bike path with crushed rock (and a few softer spots). Road bike speeds are in the low 20s for extended rides. One difference is how aero you can get; this matters more at higher speeds.