I'd agree that you probably wouldn't want to go much over 25 mph with a machine of that size. But on a 10 mile drive it would be nice keep the 25 instead of slowing down to 22 or 23 with 20.8x42's. It may not even be possible to change, I don't know enough about the system.
My experience is that all powershift 8030 series tractors will go 26.5 mph regardless of the tire size (within reason, obviously). It does this by increasing or decreasing engine max rpm in road gear to meet the proper speed for the tire. The question is, on a 9870 with ProDrive, does the system operate at a fixed top speed or is there the capability to adjust it through the computer like the tractors? You can adjust the speed to 25 mph with the knob on the console, if the combine comes from the factory with 20.8x42's will it actually meet that 25 mph that the screen shows or will it just be wrong? Coming straight from the factory will it be programmed differently for the smaller tires?
If anyone has or has run a ProDrive machine that came with smaller tires from the factory, care to comment on this?