depends on what class you're planning on running. 17.5 is a solid choice that is drivable for most folks. if you're a lil more used to the buggy then the sky is the limit with how fast you want it. The real question on the motor is how big is your track, and will you use all the RPMs that a lower turn will give you. on a super tight technical track a 17.5 will probably out race an 8.5. But take the same two buggies and put them on something closer to 8th scale size and that 8.5 is going to make that 17.5 look slow on the straights.