Combines cylinder drive belt

Belt width shouldn't matter speed wise...a wider belt will just set deeper in the driven pulley, negating a higher set in the drive pulley. Check your drive pulley, to see if it running all the way in for maximum speed (I imagine dirt could build up between these sheaves). Also check your driven pulley for any binding that would not let it run out to the maximum setting.

Belt width has more to do with speed range, but does contribute to top speed. We used to change our belt to maintain tension at lower speed ranges like 350 RPM, which is necessary for white pea beans or dry buckwheat. Dirt buildup can occur between either pulley in the groove where one sheave goes inside the other. Sorry, I don't recall the width of a new belt. 1150 RPM is about the max speed for this system with new belt. That was a nice feature on the MF 550 - you could adjust the belt tension, those belts could be run for many hours before range was affected.