"Based on a minimum tank size and fill up of 50 litres. Comparison between a standard gasoline and that same standard gasoline containing our instantaneous fuel economy formula; urban cycle comparison between a standard diesel and that same standard diesel containing our instantaneous fuel economy formula. Actual savings may vary according to vehicle, driving conditions and driving style."
In other words, you need to adopt a more fuel-efficient driving style in order to make savings. You will also need to empty all those weighty items, ensure tyres are properly inflated, ditch the roof rack etc etc. All things which will save you fuel anyway.