"O. Sami Saydjari" wrote:
My POH says that I have 72 gals of useable fuel (Piper Turbo Arrow III).
How do I really know if it can burn all 72 gals before running out?
Could it be 70? 68? Does the number change if I am climbing,
descending, cruising, or in turbulence? I would imagine it would be
affected by attitude.

The answer you seek is found in part 23, which has to do with
certification of aircraft. Most, but not all, aircraft flying today
were certified under part 23:

23.959 Unusable fuel supply.

(a) The unusable fuel supply for each tank must be established as not less
than that quantity at which the first evidence of malfunctioning occurs
under the most adverse fuel feed condition occurring under each intended
operation and flight maneuver involving that tank. Fuel system component
failures need not be considered.

So, yes, you can use all 72 gallons. In reality, of course, it's really
36 gallons per tank. Having one full tank doesn't mean you can run the
other one below the usable level and still draw from it.

Notice, however, that this is only true for "each intended operation".
Some POH's contain warnings like "turning takeoffs prohibited", "avoid
extended slips with fuel tank less than half full", "takeoff prohibited
using a tank less than 1/4 full", etc. If your POH says things like
that, then those are not "intended operations", and there's no promise
that you'll still be able to draw fuel while doing those things.

I have come to understand that the fuel flow gauge and the fuel quantity
gauges are highly inaccurate. It sure would be nice to know, with
pretty high confidence, at any given time, exactly how much fuel you
have and how many more minutes before your airplane becomes a glider.
Does someone make reliable gauges of this sort for GA aircraft?

The best way I know is to know how much fuel you started with, know what
your fuel burn is (from careful pre-flight planning, power settings, and
experience), and keep track of the time with your watch.

The next step up is to install one of the electronic fuel computers
currently available for many GA models. These things measure fuel flow
quite accurately (much more accurately than the analog gauge on your
panel, which is really a fuel pressure sensor). They also typically
have some convenient totalizing functions, and often interface with a
GPS to tell you things like estimated fuel remaining at your destination.

Is there a rule of thumb for conserving fuel in getting from point A to
point B. I presume one uses the "Best Economy" settings at 55% power.

Often that's the answer, but not always. Flying with a tailwind, you'll
cover more ground per gallon at a lower speed. With a headwind, you'll
want to fly at a higher speed. How much faster or slower? There's
rules of thumb (add half the headwind, for example), but the real answer
is to sit down with the performance charts for your plane and an E6B and
work out some sample scenarios.

Of course, one of your pre-flight planning tasks should be to look at
the winds aloft and see which altitude will give you the most favorable
winds. Most GA planes operate in such a narrow altitude band that it
often doesn't matter too much which you pick, but you said you're flying
a turbo. Your service ceiling is probably in the flight levels, so
you've got a lot more possibilities than most people.