Fuel Cost Calculator

Speed costs more than people realise

The dominant force opposing a car at motorway speed is aerodynamic drag, and drag increases with the square of velocity while the power needed to overcome it rises with the cube. That relationship is why a modest reduction in cruising speed produces a disproportionate saving in fuel. Dropping from 130 to 110 kilometres per hour on a long motorway journey typically cuts consumption by something in the region of 15 to 20 percent, while adding only a handful of minutes to the trip. It is one of the few places in everyday life where the arithmetic is this favourable and this widely ignored — most drivers will spend a great deal of effort hunting for a cheaper pump and none at all reconsidering the number on the speedometer.

Comparing efficiency figures fairly

Miles per gallon and litres per 100 kilometres are not merely different units — they run in opposite directions, and that inversion breaks people's intuition badly. MPG is efficiency, so higher is better; litres per 100 km is consumption, so lower is better. Worse, MPG is non-linear in a way that misleads: improving from 15 to 20 MPG saves far more fuel over a given distance than improving from 40 to 50 MPG, even though the second looks like the bigger jump. This is precisely why most of the world reports consumption per distance rather than distance per volume — it scales linearly with fuel used, so the numbers can be compared and added honestly.