We use somewhere between 3.5:1 and 4:1 (cant remember exactly) on big nipper becasue we only use 75mm dia wheels. Gordon is right if you use smaller wheels then you can use a smaller gear ratio with gears at smaller rated torques, the gears/sprockets and wheels are all smaller and therefore save weight but at the expense of having less surface area in contact with the ground, so less grip, which has to be compensated for in other ways.