Originally Posted by wagathon

The usual state of things is that the riding position on a mtb will be more upright than on a touring bike. Even without head and sidewinds, the energy required to overcome wind resistance at higher road speeds is . . . high!

All things being equal, a tourer will go faster on a road bike, given the same amount of energy. As far as how measurable the effect of wind resistance is, just imagine that--or so I'm told--that the force required to overcome wind resistance increases at a 3-fold rate to the increase in windforce.

Imagine also that you're riding a loaded tour bike against the wind with 50 miles to go before you reach a particular campsite--and that you want to keep up with other riders that are trying to get there before dark--you'd be in the drops of a road bike and peddling in a more efficient riding position than you'd be able to do on a mtb.

Mtbs are optimized for riding and climbing dirt trails--you give up something: efficiency at higher effective wind speeds.