That will depend on what else your going to do to it. if you have the stock tire size then i would say 13 or 15. just keep an eye on the ESC and motor temps. if it's to hot to touch then your going to fry something. get a few pinions to keep on hand so if you notice it getting hot change it. if you get bigger tires then you'll have to change the pinion, spur or both to keep the temps down.