Fuel gages can also be made inaccurate by fuel tank deformation. The
Aeronca Sedan on floats I was flying 40 years ago had fuel cells, which
I didn't really understand the consequences of at the time. It also
had sight glass gages directly into the two wing tanks. After all,
what could be more accurate and reliable I thought.........

My cross country aerial adventure with my then new girl friend started
out by a magneto failure (stone dead) discovered at altitude on the
outbound trip. That got fixed by a local tractor magneto repair shop
(Yeah, he'd seen that mag used on old Allis Chalmers - but they were
considered quite troublesome). It needed a new coil for $3.00.

But the old bird also had leaky fuel caps and possibly a plugged fuel
vent system. At any rate on the return leg the fuel levels still
indicated nice and high while cruising along. The fuel cells were
collapsing, while those wonderful gages showed lots of fuel on board.
I'd swear that thing went from 1/2 tanks indicated to a complete engine
stoppage in 5 minutes. We deadstick landed in a swamp and drifted to
shore, up to a very surprised farmer's house to ask for fuel.

It was quite an adventure. I understand it is now an AD on Aeronca
15ACs.