There's another way to check for slippage in your counter system. The counter should record 23.23232323...(I think that's the number) counts per wheel revolution. Note the count on your counter when the valve stem is even with the fork. Go for a multi-mile bike ride, and when you return note the count again when the valve stem is even with the fork. Subtract to get the counts for your ride and divide that number by 23.2323232323... If the result is not VERY close to a whole number you've got a problem.