Its not as easy as just adding a tooth or taking one away. You have to divide the front by the back which gives your ratio eg 16:12 = 1:1.333 (3dp). 18:13 would give you 1:1.385(3dp) 18:14 would give you 1:1.286(3dp) I've had the same problem, just run 18:12 and it'll work fine but won't feel the same as 16:12