As petrol passes a euro a litre, manufacturers' claimed consumption figures are being challenged. The two Japanese makers of clean energy-saving hybrids, the Toyota Prius and the Honda Civic hybrid, are accused of exaggeration.
Consumer Reports, the leading US agency which tests products and advertising, says Honda's claims of 47 miles per (US) gallon for city driving, don't match exhaustive tests showing it gets just 26. For the Prius, the only dual electric-petrol car available here, figures were 60 claimed and 35 achieved.
"I'm not saying hybrids don't have good fuel consumption," says David Champion, chief tester of the independent agency, "but people are buying them based on exaggerated claims." He says his organisation buys its own cars for testing, does more rigorous analysis and gives more realistic results than those of the US Environmental Protection Agency, on which makers rely.
After the release of the Consumer Reports analysis, CBS news reported on a Civic hybrid owner getting a 30 per cent shortfall on the mileage he had been led to expect.
This was mirrored by a British motoring journalist. "Toyota claimed 57.6 miles per (imperial) gallon, but I could only achieve 38.1mpg," says Neil Winton, a former Reuters journalist, who now tests new cars for British publications. "That's an almost 34 per cent shortfall."
For conventional cars, Winton's figures show smaller but significant shortfalls - 23.8 per cent for the Honda Jazz, and 23 per cent for the BMW 330d and the Audi A2 1.4 Tdi. He says motoring journalists need to test cars for longer than the usual week-long test period.
An EU directive regulates how consumption is measured and presented in three categories: city, country and a mix of both. "We don't take the figures at face value," says Rob Aherne, editor of What Car? magazine. "They are a valid and independent way of comparing consumption, but the results obtained in laboratory conditions often do not reflect the reality of city driving.
"Our experience with a Prius on long-term test has been more positive. Initial figures were disappointing, but they have improved. Perhaps with more miles on the clock, the engine is loosening up and becoming more efficient."
Toyota Ireland says: "The official fuel consumption for Prius was homologated by an independent accredited agency in the normal manner used by Toyota and other motor manufacturers. Irish Prius owners have regularly reported fuel consumption of between 55 - 58 mpg."