Does the text output reflect what you would expect? Your fuel system code only sets the max value of the slider if startFuel > maxFuel, are you setting the values in the editor for when that condition isn't true? Also in ReduceFuel you set startFuel to be the deltaTime * rate. If you are going at a constant speed with a constant frame rate then you're just setting startFuel to be the same value so you'll see no, or little change. You probably mean
startFuel -= Time.deltaTime * fuelComsumptionRate;
The "-=" is shorthand for
startFuel = startFuel - (...)
so you are subtracting the fuel rate from the fuel value which will see your fuel drop in relation to the speed.