The tank gauge tells you how much pressure is stored. The outlet gauge tells you what pressure the regulator is sending to the tool.
Tank pressure gauge
On the DWFP55126, DEWALT calls this the reserve air pressure in the tank. The compressor automatically stops at its cut-out pressure and restarts when tank pressure falls to cut-in pressure.
Outlet pressure gauge
This shows the regulated pressure available to the air hose and tool. It should be set to the tool manufacturer’s required working pressure, not automatically to the maximum number the compressor can produce.
Regulator
Turn the regulator to raise or lower outlet pressure. On the DWFP55126, clockwise increases pressure and counterclockwise decreases it.
Why the outlet gauge can fall while the tool is running
Some pressure drop is normal when air starts flowing. A large drop can also point to a regulator setting, restrictive hose or fittings, leaks, or a tool asking for more air than the compressor can continuously deliver.
Use the pressure-drop guide if the tool feels weak as soon as you pull the trigger.
Do not adjust safety devices to chase more pressure
The pressure switch and safety valve are not tuning knobs. DEWALT says the pressure switch cut-in and cut-out settings are factory set and warns against altering safety-valve behavior.
Sources used for this answer
Back to class: Air Compressor Class.