vehicles.
That wouldn't work for all vehicles. My own vehicle does about 3500RPM on the highway, at roughly 78MPH and 42MPG. If I drop to below 3000 RPM (slowed by traffic, too many cops around, etc.), I'm lucky (very lucky) to hit 35MPG.
While I haven't always paid close attention to RPM, I have noticed that a few vehicles (including my current one) I have owned in my lifetime had a sweet spot (for the engine) that would greatly increase fuel economy. In all the vehicles I have observed this, that maximum fuel economy point was somewhere over 70MPH. In all cases, the best fuel economy was achieved while the engine was running (relatively) high RPM.
Whether you think of it as limiting speed (like say a 55MPH speed limit) or limiting RPM (like say a 3000RPM upper limit), either way could BACKFIRE on you, if you are trying to save fuel. For your car, it might work. I've already observed that my own car uses more fuel below 3000 than it does around 3500.
If you want to know why some cars get better fuel economy at slightly higher engine RPM, look no further than the brochure used by car dealers to sell your particular car. Just about all of them have horsepower and torque ratings listed for a corresponding engine RPM range. You'll note that most engines need to be running at a rather high RPM to run MOST efficiently, though some of them also offer lots of "low-end" torque. In other words, if you want the best performance out of your engine, MOST of them will need to be run a bit closer to (redline) than (idle). The sweet spot for many cars seems to be somewhat above the halfway point. Or if your redline is 6000, the maximum fuel economy might be somewhere above 3000. Much faster OR slower, the engine will be working harder, burning fuel faster. -Dave
