When I started commuting to work I was riding my Giant MTB with knobbies 12.1 miles in 40:09 minutes. At the time I was just riding for the hell of it but as time went by I wanted to go faster. I put 1.70 slicks on it and wrecked (broken nose) on the way home on the first day. The wreck was caused from going so much faster when I entered this one tight turn that I had been through many times before. With the slicks my best time was 34:12. In a quest to go even faster I bought a road bike and my best time so far is 31:03.
I have decided to start riding the MTB more often when I do get to ride again to see if my time will improve. I have been out-of-commission for two weeks so far due to rotator cuff surgery.