Well, my feelings on this subject are mixed. Your Otto Cycle engine in your auto-tranny equipped vehicle will be most fuel-efficient in the powerband range of certain RPMs. SO, if you know how to find that fuel-efficient point*, you can try to run your engine at a certain RPM to maximize fuel economy. Obviously, if you don't have a tachometer, you can't do this. But then again, for some vehicles, it doesn't matter whether there is a tachometer or not. Take my wife's car for example. I ran the numbers to determine where it would be most fuel-efficient. It should be most fuel-efficient from 3200-3600RPM. Problem is, I discovered that 75MPH was somewhere below 3000RPM with overdrive enabled. So to get to 3200 minimum would be ummmmm, not a good idea. With overdrive OFF, RPM jumped up above 4000. Basically, I discovered that my wife's car was geared incorrectly. So in the case of that specific vehicle, a tachometer is useless. I can't drive it legally AND keep the engine in the powerband. -Dave
* Various sources say 40% of redline, 40-45% of redline, or 60% of RPM at maximum horsepower RPM. In the case of my wife's car, this means it will be most fuel-efficient in a range from 3200-3600RPM.
