I agree with the guy who said class ratings are not so much to do with overall capacity.

Seems like HP, hopper capacity, unload times are what they go by.    Looking at JD's combines everything is the same aside from the 3 items mentioned in the last sentance.  Not saying the overall capacity's are the same, but Id put a dollar on a chipped out 9660 keeping up allongside a 9760.    A dollar is about all I have to spare!

Good Luck