I have 2  1988 8820T2s One has  a 5 yearold maybe 500 hrs rebuilt hydro.  Will easily run 6.5 in 3rd gear. Other 8820  new belts reversed pressure valves  same size drive  tires will only go about 5.8 in  3rd about a 1.5 slower in road gear also, used to run faster about 3years  ago.  I thnk  I have a tired  hydro in the  slow combine, any other ideas. The speed matters because this harvest we used Shelborne Strippers heads and found we can harvest at 6+ mph .  If any body knows of a hydrostadt or a iar foil chaffer I would be interested.