let's look at your '36 vs '20 suggestion. The '36 head has 80% more capacity than the '20. I highly doubt that the '20 operator is going to be going almost twice as fast as the larger one. If that machine was running 5mph the '36 head would run at 2.7 mph to be equal. Human nature tells me that the operator will push the larger head more to the max than one with the smaller head. So he will probably be less limited by ground speed, while the '20 speed is limited more by ground conditions. This being said, it is likely that the larger head will send much more grain through the combine. Augers, sieves, concaves, rasps, grain pans, etc. will all take much more wear and tear with the extra grain. I agree with Jake that it is wise to consider that a larger head will wear out a combine quicker, whether it is a corn or grain head.