Actually, the most efficient transmission I ever ran was in the old Schumacher Top Cat and Cougar series... It had a pully for the Diff and a pully on the top shaft, with a wide belt connecting them. This eliminated the weight, etc. of the idler gear, making the tranny VERY light and responsive. It worked GREAT for dirt oval, but for Offroad, the tranny was TOO responsive -- if you got off the throttle, the car didn't want to roll through the turns (like a gear tranny does)...
"If you cannot win, make the one ahead of you break the record."
Biff Racing Team #420 (Ah... The "good old days"...)
The local indoor Offroad Track: The RC Race Barn
TLR22-3.0, TLR22T-2.0, TLR22SCT-1.0