It's not the RPMs as much as the comfort zone of the engine. Throttle position is key. If your gears (trans, tire size, rear end, or OD) make it necessary to be deep into the throttle to go down the road you won't get good MPG at any RPM. The good old vacuum gauge is a real help in finding the zone. The most amount of work with the least amount of throttle. The combinations to get that won't be the same every where or for every use, that's the real world aspect.
"I wonder if God created man because he was disappointed in the monkey?" Mark Twain
