Declared "minimum fuel"

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.

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?

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.
Of course, that also slows you way down...but I am guessing that you
would still be better off in terms of preserving the most fuel by the
time you got to point B, right?


-Sami