Jamie,

Pain In The Asp is a real hybrid of drive systems,
First, Astroflight C40 motors driving through a Team Whyachi TWA40 3 stage gearbox @ 7.78:1 then a set of Mod 1.25HD spur gears @ 2.2:1to the rear axel (overall ratio 17.11:1)
From rear axel to center and front axels I use 2 sets of 5mm pitch X 15mm wide HTD timing belts and pullies @1:1

Not light, but incredibly strong and reliable.