how do you tell if your belt is worn. if there is no spec. on the belt, I measured the width of my belt and the width of the pulley,. and if the pulley is wider than the belt, your not going to get the best rpm. at speed, (rpm to high),. my pulley was 25 nmm. wide, and my new belt was 24 mm. wide, so I shaved the pulley 1 mm. on each side, now the pulley is about 23 1/2 mm. wide , and my rpm droped 1,000 rpm at speed, I looked at the belt with the tranny cover off, and the belt goes to the top of the pulley at 6,000 rpm. befor it would go to 1/4 inch from the top at 7.000 rpm. at 6,000 rpm. I go from 60 to 65 mph. and at 6,500 rpm. I go 70 to 75 mph. Mitsuboshi spec . was 25.2 mm. x 1004 mm. with 30 degree pitch ,. on both of the new belts I bought the belts are 24 mm. wide, x1004 mm. thats why I shaved the pulleys,. I ride a 300i down town 2011 now have 25.000 miles on it,. and I just changed the belt it was starting to have little cracks between the bumps when I squeezed the belt, and I was getting tired of checking it every oil change,.