I run a 45:17 or 45:16 and thats sufficient enough for me. The 45/17 is around 70 gear inches and I find it a lot easier to slow down, skid, ect with that. Plenty of speed for me even on the flats since I'm not looking to be speed racer or anything. When I first got a fixie it was 48:16 and that was FAST but scary as hell trying to stop it. Start low and build up.