Don’t Fill Up Your Tank
The more fuel you put into your car, the more weight you inadvertently add. So it makes sense that the extra pounds will make your consumption heavier. As much is demonstrated by Formula 1 cars, which sometimes reduce their fuel loads to pick up speed more quickly. It is also true: more weight does equal higher consumption. But a F1 car needs every edge it can get and the fuel reductions are about speed, not saving fuel. The average car does not gain any real advantage. According to the Environmental Protection Agency, every 45 kilograms added to a car reduces its efficiency by a 1% or so. Petrol weighs a bit less than a kilogram a litre. You would need to fill a ninety litre tank in order to use 1/100th more fuel than you would be on half a tank. On normal cars it wouldn't be noticeable.