Try this: http://www.midnightcentury.com/supercruise.jpg Rider aerodynamics matters as your speed gets into road-bike territory. If your bike has a short reach, this may not work so great... mine is nice and spacious:


I broke this one, but you get the idea

I've turned out some pretty fast rides on my XC bike, such as 20.0mph / 32kph average on a stop-&-go shop ride the other day... http://app.strava.com/activities/13243193 That was an OK ride considering I'm out of shape lately. At several points I was actually gearing-limited with a 44x11. On level ground with neutral winds, in good training, I gravitate towards the 44x12 at about 86-88rpm on 26" wheels for a fast tempo cruise.

Speaking of 44x12, most mountain-bike cassettes don't have a 12. If you're doing a lot of road cruising, consider switching to an 11-32 cassette if you presently have an 11-34 or some other combo that doesn't have a 12.