The Chieftains I used to fly on a casual basis for the old ASA back in the early noughties had fuel guages that were surprisingly accurate!
One one occasion, at the encouragement of some of the Line Pilots I decided to run the 'Auxes' down until an Engine surged. A ferry flight, only me on the A/C. Each engine selected about 15 minutes apart.
At two full needle widths below the empty indication on the guage with my hand on the selector and auxillary pumps on my nerve gave out, and I swapped back to the mains whilst the engine was still running sweetly!
I was assured that the Mains guages were of the same accuracy, but the only way I would trust that statement would have been a calibration done by the LAME's.
On the other hand; Early model Cessna 402's guages were, how shall we say, not as accurate! Especially on low readings.
And we shall leave it there I think!