I currently ride a Trek 3900, hardtail, knobby tires. There are two typical rides I do on the weekends. One is mostly crushed limestone with a little pavement. The other is all pavement. I usually go for about 25-30 miles on either, my average when I get home is usually 13mph, a little more on the pavement. If I switch to a road bike, what sort of distance increase or average increase can I expect to see?