Final answer:
The thermometer should be left in the fuel for a few minutes to ensure an accurate temperature reading.
Step-by-step explanation:
The length of time that the thermometer must be left in the fuel to ensure an accurate temperature reading when manually gauging a tank depends on the properties of the fuel and the surrounding environment. Generally, it is recommended to leave the thermometer in the fuel for at least a few minutes to allow it to reach thermal equilibrium with the fuel. This ensures that the thermometer accurately measures the temperature of the fuel.