old cog is a surly and was on some brev rear that a friend gave me but he had ridden on it for at least a year or 2, was still fine but wanted a replacement and reason for the 18 tooth is because the mechanic yesterday convinced me to buy they 18 tooth and cant really return i assume. especially after riding for an hour or 2 on it. and dont trust the current stock cranks on the flite so wanted to upgrade to the all city. but guess when i change the cranks i can grab a 16 tooth cog and sell the 18 somewhere or something. 46x16 will work better or should i just go back to 17? know its preference but whats the skid patch count on either?