Vacuum advance tuning to maximize cruise MPG?

Have you re-adjusted the idle speed and the idle mixture? That may help with coast down. Typically set lean mixture to curb the idle speed 30 to 50 RPM from max. Too lean results in a miss and increased HC. It may be necessary to re-adjust speed and mixture since they are inter-related. The throttle stop (idle speed screw) must stop the throttle plates from contacting and wearing the bore. Plate alignment and throttle shafts need to be free of wear related leaks, or it is difficult to meet desired idle speed with advanced timing.