If an airplane does not have accurate fuel gauges then it is not
airworthy. Knowingly climbing into an aircraft that you know has
inaccurate fuel gauges is both dangerous and illegal. That is why you
check them during preflight. From FAR 91.205:

Horse****.




(a) General. Except as provided in paragraphs (c)(3) and (e) of this
section, no person may operate a powered civil aircraft with a standard
category U.S. airworthiness certificate

...
(9) Fuel gauge indicating the quantity of fuel in each tank.


To what precision and accuracy and where defined? More than half but less
than full? More than empty but less than a gallon? More than ten gallons
and less than 11? Cite accuracy regs, please.

Jim