I've been told that for the best fuel economy and performance, your rpm at highway speed should be at the rpm where your engine has peak torque. Gas and Diesel have different torque curves, with gas being a more sharp peak in the curve, at a little higher rpm. For my gas engine, peak torque is at 2000 rpm. So, with me running about 2200 rpm at 60 mph, I guess that would be about right. I find I get 10.9 mpg -USA Gallon, towing the 5,000 lb 22FB trailer, hand calculated.