Football fans who deck their cars with red-and-white England flags are sacrificing more for their country than they may have imagined.

Tests have computed that the symbols are costing their users a grand total of 4,500,000 gallons of petrol, thanks to aerodynamic drag.

The national bill is more than £17m, and each individual driver is losing 1.5 gallons, or more than £5.50. The figures come from calculations by Peter Stevens, the designer of the McLaren F1 roadcar, who organised a test drive in the Motor Industry Research Association's wind tunnel at Nuneaton in Warwickshire.

The tests, commissioned by Top Gear magazine, showed that at 60mph a car with two flags loses four brake horsepower - a 2% drop in engine efficiency. The overall figure was calculated on sale of some 4.5m flags to an estimated 3 million drivers, whose normal travelling was averaged at 10,000 miles annually at 28 miles a gallon.