General Cycling Discussion - Trail vs. Road Riding

I was just wondering how much harder it is to ride on a smoothly graveled bike trail compared to riding on a blacktop road. I have never ridden a ‘century’, just 80 miles on a smoothly graveled rail-trail with a mountain bike with aggressive tire treads. Do you think this would be comparable to riding a ‘century’ with a road bike on equally level blacktop terrain? It seems to me you work significantly harder on gravel compared to blacktop.
Would a change to a less aggressive tire make a significant difference in my speed? Since I primarily ride on rail-trails with some road riding I bought a Cannondale mountain bike (for durability-5600 miles without a flat so far), though the trails I ride are pretty smooth. I would like to improve my speed, so I guess the tires would make the biggest change to my riding. I'm sure changing to a road bike to ride the trails would make a big difference, since the bike would be a lot lighter than what I have now. Can I get a reasonable increase in speed with just swapping tires?

I don't know about gravel, but ON the road you'll go significantly faster with slicks than with knobby tires. I run 1.5" semi-slicks on my rigid MTB and I can do 20mph pretty easily.

I've done a couple solo centuries on the local rails-to-trails (Katy Trail). I notice a difference between fresh, smooth blacktop and anything else. The Katy trail, while relatively hard-surfaced, has a very irregular finish to it, and seems to slow me down significantly. I don't know if a smoother tire would make much difference on this type of surface, but a lighter, higher-pressure tire would help a little. An ultra narrow tire could increase handling problems in areas of loose gravel or areas of erosion.

On asphalt, high pressure slicks have much less friction, hence allow you to go faster. On hard packed gravel, it depends.

I have ridden a few with my usual touring tires: 700x32 or 700x37 slicks (Top Touring 2000)
- With a very hard packed trail, it may be almost as easy as on asphalt. On Le petit train du Nord, I might loose 1-2 km/h (i.e. 18 instead of 20 km/h average).
Wetness has a more drastic effect on packed gravel than on asphalt, however.

- It doesn't take much smoothness to slow down the rider by an extra 2-3 km/h.

- On medium-hard gravel, wide slick tires (700x37) allow a decent speed, whereas narrow tires (700x28 or less) dig their way in.


One aspect where asphalt may be problematic : when there is a poor base and the asphalt has lifted, cracked, etc., cracks in the asphalt may slow you down more than ondulations on hard packed gravel dust.