Singles pack the biggest power, but tend to act like a tempermental race engine - it all comes in with a rush at the top end. Great for fast open tracks, but too juicy and scary when geared for a tight track.
Doubles pack the biggest torque, but don't have the best top end. Great for medium tracks, and usable on tight tracks with a little lower gearing. Best where there is a lot of acceleration out of low/medium speed corners.
Triples are really smooth, but lack both top end, and low-down torque. Best where there is not much change in speed across the whole track, or where the twisty bits follow one another, so there is no big change in speed required in a hurry.
Quads, quins and sixtuples were a nice idea - and that's all!
the best all-round wind is a double, and they will stand gearing errors best
Fusion Hobbies / www.horshamrc.org
The wife stops me being sane and the r/c stops me from going insane....