The fuel gauges would have been reliable when first certified.
FAR 23 lists the certification requirements for small aircraft -
23.1337(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. In addition:
(1) Each fuel quantity indicator must be calibrated to read “zero” during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under §23.959(a);
So unless you lift the tail of a 140 to a level attitude there is no requirement for the fuel gauge to show you useful information. If you weren't taught that in primary flight training I'd say your instructor was negligent.