I think that beta-caratene might just do the trick.
Three things -
It will make headlights appear brighter because eating carrots improves night vision, well known fact discovered in WW2.
The other effect that beta-caratene has is on the motor-neurone system to the right foot. It weakens the amount of pressure that you can exert on the accelerator pedal. Hence the improvement in fuel consumption.
Third is that it affects the sensory balance system by which one determines motion. It makes you think that you are going faster than you actually are.
So, you are not supposed to put into the fuel tank, you should drink it instead.
But do remember to dilute it with the appropriate volume of snake oil - vitally important!