I have a 1990 Amberjack. At dockside...

[Fishtank]

I have a 1990 Amberjack.  At dockside my fuel gauges show SIGNIFICANTLY lower fuel reading than when running.  

Which reading is correct?

Anyway to get an actual visual measurement of fuel in the tanks?

[wingless]

The attitude of the boat changes when at rest compared to underway.

The fuel in the tank remains level in both conditions (neglecting sloshing around).

The fuel gauge measures the top level at the sensor location, so it will follow those changes.

The actual measurement is at rest.

The pickup will be at the aft end of the tank, the sensor is usually forward or mid tank.

[Fishtank]

The Wingless but I don’t quite follow your reply. If the attitude of the boat rises underway, it would seem fuel would raise the level in aft of tank. You mention a sensor and a pickup, aren’t these the same thing?

[wingless]

The integrated level / pickup is common on cars, but on boats w/ large aluminum tanks, there is a fitting on the rear for the fuel pickup. It is a part like this, threaded into the top.

https://www.suremarineservice.com/Heat/Fuel-Tanks-Fuel-Pick-up-tubes-and-Accessories/W005-913-1.html

The level sensor is at a different place and it is a part like this, threaded into the top.

https://www.suremarineservice.com/Heat/Fuel-Tanks-Fuel-Pick-up-tubes-and-Accessories/TFGASSL-13.html

There are also fittings for the vents and the returns, as-required.

[Fishtank]

Thank you