I'm really old, so this thread is providing an enormous amount of entertainment. ECOnomy in driving is the purpose of MDS, no? OK, think of some of the really problematic drivers of a Prius that you've seen. What are they doing, going so slow? Watching the indicators on their instrument panel. I don't know exactly what they see, but here's what's going on:
When your engine is idling, it pulls maximum vacuum and uses the least fuel. As soon as you start nudging the throttle, the vacuum starts to drop. More throttle = more fuel, so less ECOnomy. At the same time, less vacuum. If you had a simple vacuum gauge attached to the engine, it would provide the same information or turn on a light, or tell MDS to start or to stop. Years ago, it was possible to hook up a vacuum gauge and let that tell you what gear to choose and how much throttle to give it in order to maximize your gas mileage. Also,
Early automobiles had vacuum-operated windshield wipers. If you floored the gas pedal, the windshield wipers would stop wherever they were in the stroke and stay there at least until you let up part way on the throttle.
