if youve added anything to that bike (even pipes) id run a 15, bone stock the 14 will do, the key to riding trails (or anything really) is traction and acceration, a 12 just means unless your in 4th gear every time you gas that bitch its spinning,a banshee with pipes will pull a 15 with 20" tires all day long, that 12 just means the bike will never really pull, it will just hang and float through all its gears, maybe a 12 would make 6th pull to about 60mph....bottom line if your geared to tall you might have to feather the clutch, but geared way to low, the motor will spin like a son of a bitch and it will feel like your standing still