last time i checked, most fuel tanks use a buoy type measuring system.
when you accelerate, weight shifts to the rear of the vehicle, and hence the fuel shifts to the rear. when you travel at constant speed, fuel sits level.
so when you accelerate hard, weight shifts to the rear, the top line of the fuel is hence on an angle instead of being level, causing the buoy to drop down a little (30L of fuel sitting on an exponential like curve would probably have the meter reading something like 20-25L instead, depending on the acceleration of your car).
then when you cruise at a constant speed, all directional forces essentially balance out and the fuel sits level again, so your fuel meter reads the fuel level correctly
even if im wrong about the buoy type system, the physics match. and although my explanation may suck, those who have watched initial d should get the gist of it by watching the way the water in a cup behaves.
