The fact that they aren't knobbies is a big factor. I guess i jumped to that conclusion on my own, so I apologize. However, that is still a pretty fast average even on slicks. I run slicks on one of my MTB's on occasion. I don't recall if I ever averaged that fast or not.

I do know that 17 mph is the lower end of my average on my road bike. IOW, if I average 17 mph I know that conditions were not ideal, hills, wind etc. I usually average around 18-19 mph on the roadbike with perfect conditions. I averaged 18.9 on my last century, but that was with a stiff tailwind most of the way.

The other day I averaged 16.5 on the roadbike with a 35 mph North wind.