Over the years I've owned my current ride, I have observed that it is most
fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
or lower. My experience with my current vehicle is similar to several other
vehicles I've owned. In every case, I've found that the best fuel economy
is achieved somewhere above 70MPH. In the case of my current vehicle,
3500RPM in top gear equates to about 78MPH. Now I'm sure someone will state
the obvious, which is how do I know that my tachometer and speedometer are
accurate? I don't. But the bottom line is, in top gear, I can achieve
BETTER fuel economy at slightly higher than normal highway speeds.
Actually, the better fuel economy is due to slightly higher engine RPM, and
the higher ground speed is COINCIDENTAL.

I've always known that every engine has a "sweet spot", but didn't know the
exact scientific explanation for my personal observations until just
yesterday. During research on various engine types, I happened to discover
that Otto Cycle engines are most fuel-efficient at RPMs near (not
necessarily at, but near) 40% of redline. I had no reason to disbelieve
that particular scientific fact. I did the math for my own car, and found
that (surprise, surprise) I SHOULD achieve maximum fuel economy at ABOUT
3200RPM, with my own Otto Cycle engine. This agrees almost exactly with my
own observations that 3500RPM (43% of redline) is very fuel efficient, and
3000RPM or less (37% of redline or lower) is not as fuel efficient.
Obviously, my engine is very fuel-efficient near 40% of redline, AS IT
SHOULD BE, and it is less fuel-efficient at lower RPM levels. Note that I
had no idea that my engine SHOULD be most efficient at about 40% of redline
before I observed that it WAS most efficient at 3500RPM. So the results I
observed were not results that I "expected" to observe. The facts are what
the facts are, and now I have the scientific explanation stating YES, it
should be that way.

From what I could gather, the reason for the lower fuel efficiency below
~40% of redline in an Otto Cycle engine has to do with valve timing and
other aspects of engine performance. Most Otto Cycle engines have fixed
valve timing, so the maximum fuel economy is achieved at a certain RPM, and
if you run the engine higher OR LOWER than that RPM, your engine is not
achieving maximum fuel efficiency. Some engine manufacturers have come out
with variable valve timing Otto Cycle engine designs, with limited success
in improving fuel efficiency at lower RPMs. I gather that the success is
"limited" because fixed valve timing isn't the only factor making an Otto
Cycle engine most efficient at a certain RPM.

Anyway, now that I know WHY I'm seeing fuel economy peak at certain RPMs, I
know exactly what to look for in my NEXT vehicle. IF it has an Otto Cycle
engine, it WILL have a tachometer, or I won't buy it. During the test
drive, I'm taking it out on the highway to see how fast the car is
travelling at 40% of the engine redline. The way I USUALLY drive, a car
would be most efficient for me if it hit 40% of the engine's redline between
70 and 75MPH. Ironically, my current car is designed for a less
conservative driver than its current owner is. While I do get close to
80 sometimes, the vast majority of my highway driving is in the range of
70-75MPH, so that is where I need a car to be most efficient. SIMPLE,
right?

So if you want to find a car that will be very fuel-efficient for the way
you drive, simply look at the tachometer. Wherever it ends, muliply the
highest number by (.4). This will give you a good estimate of the most
fuel-efficient speed (RPM) of the engine. Then take it out on the highway
and see if the engine will run at that speed (RPM) at the speed you normally
drive on the highway. If the engine RPM is too high OR too LOW at your
normal highway speed, you might want to find a car with an engine that is
better designed for the way YOU drive. That is, if your goal is to find a
fuel-efficient vehicle. -Dave

On a side note, it's easy to understand why manual tranny vehicles are more
fuel-efficient for use in City driving. You have to get the engine RPMs UP
to maximize fuel economy, and you just can't do that at low speeds in a
slush-box.