Not at all. It is counter-intuitive, however, so a lot of people have a tough time believing that it might be possible, unless they've experienced it themselves. What speed will be most efficient for any particular engine depends on the design of the car and of the engine itself. If an engine designed to work most efficiently at say, 3500 RPM, is run at 2500 RPM usually, that means that the engine is working "harder" to maintain a constant speed, because it has a lot less horsepower at 2500RPM. Not that it's bad for the engine, and not that the engine is wasting fuel. It's just that the engine won't use significantly more fuel at 3500 RPM, because that is the optimum speed it was designed to run at. And at 3500 RPM, the vehicle will be 'pushed' FARTHER on almost the same amount of fuel. The result is that many people do see REAL improvements in fuel economy by driving just a little faster. But this varies from car to car. As I posted earlier, several vehicles I have owned were most efficient at ground speeds above 70MPH. My current car gets 35MPG at ~65MPH. If I average ~78MPH, I get 42MPG. Yes, there will always be someone who screams, BUT WHAT ABOUT INCREASED WIND DRAG? Simple. If your engine is nearer it's optimum operating range, it has more than enough EXTRA horsepower to overcome the increase in wind drag, making wind drag less of a factor in fuel economy.
Many, MANY people have experienced this first-hand, and know that their own cars get better fuel economy at certain speeds, and that those most efficient speeds are often somewhere above what OTHER PEOPLE might GUESS would be the car's most efficient operating ground speed. It is not unusual AT ALL.
If you want a good demonstration of why a car engine might burn less fuel to cover a fixed distance at a slightly higher RPM, try the following:
Find that ~50 pound barbell gathering dust in your basement. Set it on the floor. Now, lift it up above your head quickly and then set it back down on the floor again. Easy, right? Now repeat. But this time, as you are lifting the barbell above your head, do it SLOWLY, counting to (say) 100 as you are lifting it, and counting backward from 100 to zero as you are setting it down again. Not so easy this time, was it? Twice, you lifted the barbell above your head and set it back down again. The barbell moved THE SAME DISTANCE, both times you lifted it and set it back down. Doing it slowly took a lot more energy, as your own body's "engine" was operating at an "RPM range" significantly lower than optimal. That's a rather extreme example, but it should clearly illustrate why engine DESIGN is the reason certain engines use less fuel at higher RPMs. (if they are pushing a car a fixed distance) -Dave
