Of course, you'd probably do a little better than that, because at no point would your car be carrying all of that gas. But the trough would have to get wider when you went uphill or had to get going at a light.
Instead of scooping up the fuel, you could think of it as laying out the fuel in a line behind the car. All the fuel starts off in the fuel tank, and slowly drains out of the car. Fuel efficiency is how large the hole is that the fuel drains out of. In reality, it gets turned into a gas and blows away, but the effect is much the same.
Not really, fuel consumption as an input takes carrying or not carrying fuel into account...same as wind resistance of the pickup or the friction of it moving through a trough of liquid or the force of the liquid moving into the void behind the pickup, etc.
