1:2 or more properly .5 to 1 . bigger on the motor than on the tx. i have seen 5 speeds run 40 with 38" tires, but for a trail runner I wouldnt go more than 25. find out the gear ratio of the tx and run it through a gocart mph calculator. I could give the formula, but it is easier to do the auto calc. you will need tire size and gear ratio. you can change ratio to find the pulley you need for the speed you want. aim for too fast and you will run out of motor, aim too low is better, your tranny will last longer.