gear reduction

hi i have a 96 foutrax 300 4×4
witch its running 27-10-12 vampire tires, but the quad i feel its to slow, my buddy running a stock 250 yamaha bear tracker its way more faster than mine. being a bigger engine
so if i want faster and powerful quad fast take off, faster running and powerful for hill climbing. what to i have to do?
smaller stck tires? gear reduction? how much gear reduction?