Depends on tire size, engine size and what you want for a top speed. Check out the speed calc here, play with different numbers and find what you like.
http://www.diygokarts.com/speed-calculator.html
To get you gear ratio divide your axle sprocket by engine sprocket (example, 60t on axle 10t on engine=6:1) Same with pulleys, say you have a 3" pulley on the engine and 6" on the transmission you get a 2:1 ratio. If you have more than one reduction, such as 2:1 from engine to transmission, then 7:1 in transmission, then another 6:1 from transmission to tires you simply multiply 2x7x6=84
and since 1x1x1 is 1 then its 84:1.
(i feel like a math teacher...)