"they operate at 3000 while belt driving the generator at 3600 RPM"
Now what you're saying may be true of direct-drive alternators, it's certainly not the case here. Since they're using a belt-drive, Kubota has the option of running the engine at any speed they choose, and they chose to run it at 3000 rpm. And I was specifically addressing Blue's point about why THESE engines are run at 3000 rpm in a belt-drive generator application, nothing else. My positions still stands: they run them at 3000 rpm to ensure they have plenty of extra power so the engine doesn't droop under heavy load.
As for fuel consumption, there's no need to speculate as to what rpm gives best efficiency and how that relates to the engine's torque curve. Just look at the graph provided by Blue-Man: The specific fuel consumption is shown as the bottom line on the graph in the customary units of lbs per horsepower-hour. It's hard say exactly where it is lowest, but it's nearly flat from 2000 to 2800 rpm. With that in mind, it only makes sense to run the engine at a speed where it has plenty of horsepower on tap; better to burn a little extra fuel than to not have enough power to pull the load at all.
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Today's Featured Article - Oil Bath Air Filters - by Chris Pratt. Some of us grew up thinking that an air filter was a paper thing that allowed air to pass while trapping dirt particles of a particles of a certain size. What a surprise to open up your first old tractor's air filter case and find a can that appears to be filled with the scrap metal swept from around a machine shop metal lathe. To top that off, you have a cup with oil in it ("why would you want to lubricate your carburetor?"). On closer examination (and some reading in a AC D-14 service manual), I found out that this is a pretty ingenious method of cleaning the air in the tractor's intake tract.
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