How much fuel is in a 18 gal tank when it is one mark above the red?

If the fuel gauge indicates one mark above the red in an 18-gallon tank, it typically means you have a small reserve of fuel left. While the exact amount can vary based on the gauge calibration, it is generally safe to estimate that there might be around 1-2 gallons remaining. Therefore, the tank could have approximately 1 to 2 gallons of fuel when the indicator is just above the red mark.