I think it depends on how good of a rider you are and how far your racing. The usual story I tell is when I was 16 on front and buddy on 1000 was stock geared, I would pull him out of the hole from a 1st gear 30 kick, then I would watch him go bye and proceed to walk away. Mind you we only had 6 hp difference. If you cant keep the nose down it doesnt do any good at all to go with lower gear ratio. Im running 17 and 39 it feels great, I can give it everything it has especially when im strapped and lowered without worrying about bringing up the front and then i depend on my hp to take care of the rest!