Mountain:
The idea here is that the larger the radius for the turn of the track creates less friction (less rolling resistance) than a smaller radius wheel.
It is one of those things that you do when you exhaust all your other gains (Unless you just plain like the looks of the big wheel kit). Clutching, and increasing HP out put would get you far more than a big wheel kit ever would. It is one of those incremental things.
YOu do lose a fractional amount of track footprint with a bigwheel kit on a given track length....
Yep they look cool, and do give you a gain, but how much is highly debateable.......