Originally Posted by gmc
The person filling it doesn’t need a gauge...
They fill it until it reaches the relief valve they opened.
The tanks only get filled to 80%.
Isn't the simple solution to fill the tank, and adjust/calibrate the gauge(s) to show it is either 80% or full (depending on what you want to call "full."
If the gauges are not adjustable, the reading on a "full" tank will tell you which gauge is most reliable.
[Although, if the gauge doesn't respond linearly, it may not be accurate at other levels. A gauge which is accurately reporting a 3/4 full tank may be out to lunch when it thinks the tank is 1/4 full. Gasoline gauges are much better these days, but I can remember when hardly anybody relied on fuel gauges. In fact, my '07 BMW R1200 RT would tell you when it was full and when it was pretty close to empty, but in between it was unreliable. We relied on trip mileage, not the gauge. The discussion boards reported a wide variation in the reliability of the gauge from bike to bike.]