[jjones752]

51/13 is 3.923, 55/14 is 3.928, so numerically very close; the biggest difference is you would have to add 4 pins to your chain or suck the rear axle in to get the right chain tension. Also a tiny bit more rotating mass, not sure if it would be enough to affect acceleration, but it's there nevertheless. Depending on whether or not you already have a 51 or 55-tooth rear sprocket you may be better off to replace your front sprocket; sprockets are sort of "consumables" anyway, you should replace them periodically. Saves chains and grief in the long run...