But carrying extra fuel adds to the weight of any vehicle, so it makes fuel-efficiency and environmental sense to carry as little as possible. Which is why motorists should take Ryanair’s recommendation on board.

Driving with a tank full of petrol means you get fewer kilometres per litre when your tank is full, it costs you money, and your car produces more pollution for every mile that you drive.

If the price of petrol dropped after, say, the first 25 litres, then it might make sense to bulk buy. But it doesn’t. Instead, your fuel efficiency drops. Put another way: bulk buying is costing you money. Better to half-fill the tank instead.