Twisted

The thing is, it's not necessarily that the lower the rpms, the better the gas mileage. There's an optimal rpm range for fuel economy. If you are lugging along in 5th at 1600 rpms, I doubt you'd get better gas mileage than you would at 1900 rpms. Keeping low rpms is good, but there's a point of diminished returns...a point at which the engine is working harder, and you have to push the gas down harder because the rpms are too low. There's a guy on another forum that was able to get 30 miles per gallon on the highway, by hooking up a vacuum gauge, and maintaining the highest amount of vacuum while driving. The idea was that the higher the vacuum, the less the engine was working. He found that, with stock gears, he was sometimes getting better gas mileage keeping it in 4th gear rather than 5th on the highway. I don't know how true that is, but it certainly stands to reason.

AdderMk2

I can certainly see how that would be true.
Some roads have only 45-50mph speed limits. Which in MY car sits you right around 2000 rpms in 4th gear, and about 1500 in 5th. so that just points towards your statement of having to make the engine work harder at lower rpms'