That little wirewound potentiometer inside the fuel tank varies
current flow to the gas gauge to indicate fuel level, and most such
systems are accurate to about 30%. That little pot makes sparks at
times, inside the tank, and isn't sealed against fuel. The theory is
that fuel vapours in the tank are too strong (more than 8:1) to burn.
Works OK unless the tank is drained and left open and the
concentrations get low enough to go boom. "Empty" tanks can be
dangerous.
Our airplanes have the same setup. Larger aircraft have
capacitance fuel level systems, with no moving parts, and are much more
accurate.
If I was making a tank for an automobile I would use plain old
hot-rolled 20 gauge steel sheet, TIG or MIG welded. Epoxy paint on the
outside for corrosion resistance.

Dan