RPM

it's a matter of what type of kind of thing you're trying to acheive... if you're 'spinning' (pedal fast / low gear) then that's better workout anaerobically (I think that's right) for Stamina/endurance - whereas pushing the big gears with a slower cadence is more for Strength/Muscle building...

I think it's horse for courses...

I've recently read about it somewhere... I think shedon brown covers some of this on his legendary website... try this.

I remember reading something a while back, maybe Bill Strickland, about cadence. He said that roadies like it fast, because they require less burst power. Less power altogether, as they only need enough power to keep a steady speed. Mountain bikers on the other hand, will tend to keep the cadence lower, sacrificing speed for power. A mountain biker always has a big pedal kick in reserve.

In any case, it probably depends on the bike you're riding, or more specific, the tires you roll on, and the surface. The fatter and more knobby, the more they will drag on the road, and thus the more power you will have to produce to keep a steady speed. I used to keep a set of wheels fitted with 1.0 slicks for commuting. Easily an average speed increase of 5-10kph with very little effort in exchange.

I find it difficult to maintain a high cadence on my mountain bikes on roads. It feels like the energy is just getting sucked out of my legs. I take it down a gear and try to just cruise, but then, I'm a bit of an old man.