The EM2000 is nice for giving super detailed info on fuel levels, and I found that fuel usage seems to be based more on motor load than on engine RPM. Leaving the 66 in neutral and throttling up to notch 8 still uses pretty much no fuel, as if the locomotive were in idle. Only a couple milliliters every second. Engines are not 100% efficient, it takes a noticeable amount of fuel to keep an engine moving quickly even if it isn't actually doing anything.