Some people say that filling up the fuel tank to its full capacity is detrimental to fuel economy. They suggest filling up just half the tank at every fuel stop.

However, none of the motorists who believe in this idea has been able to explain why this is so.

Is it because a full tank causes more fuel to flow or more fuel to evaporate? Or is it just a myth?

A fuel tank filled to the brim will not increase the fuel flow. The quantity of fuel consumed by the engine is controlled by the injection system, which is, in many ways, directly determined by your right foot.

The evaporation theory is also flawed because petrol lost to the atmosphere is, firstly, insignificant and, secondly, not a function of the volume in the tank.

Any change in fuel consumption that you detect, because you now fill only to half tank as opposed to full tank, can only be the result of weight difference.

The fuel tank capacity of an average family saloon is roughly 60 litres, which in terms of weight is about 50kg.

Hence filling to half tank reduces the load you carry by about 25kg. This might make a small difference in the fuel consumption, but nothing that can compare with good motoring habits such as keeping the engine in good health, tyres properly inflated and the interior and boot clear of unnecessary items.

Having good driving habits, such as accelerating smoothly and quickly to an allowable speed and lifting-off to cruise in anticipation of traffic ahead, will save the most fuel.

Shreejit Changaroth