I drive in the highest gear that will let me keep the car moving. If I'm just cruising, I can use 5th as low as 30mph. I like to down-shift before the car gets to 100% load (WOT) because I don't want it going open loop and running slightly rich if I can avoid it. So if your foot is on the floor, you have chosen a gear that is too high. But I don't think it's possible to do damage to the engine or drivetrain from having the RPMs too low.
The only caveat is that you may burn more fuel by accelerating in way too high of a gear with the engine turning too slowly vs. one gear lower and allowing the engine to run where it makes power more efficiently. That's mainly because you could end up trying to accelerate forever rather than getting the job done and backing off to a steady cruise.
Also keep in mind that driving in a very high gear makes fuel economy EXTREMELY sensitive to throttle position. It's going to be difficult without any kind of instrumentation, but you need to take care to make sure that your foot really is using the lightest touch possible to maintain a given speed. This could mean the difference between 25mpg and 50mpg at any given moment with little or no seat of the pants indication.
__________________
'07 Toyota Prius
|