Car economy claims show green vehicles still burn more fuel Families are being left hundreds of pounds out of pocket because of "unrealistic" fuel economy figures in new cars a report has warned. Car manufacturers often boast about the "miles per gallon" rate of their vehicles to attract cash strapped motorists fearful of ever increasing petrol and diesel prices. But figures released by the consumer watchdog "Which?" show the difference between claim and reality can be as much as 20 miles to the gallon. The 'greener' the car, the bigger the gap, with diesels faring worse than petrol engines, a report warns It means cash-strapped drivers can end up nearly £350 a year worse off than they expect to be. And the "greener" the car, the bigger the gap, with diesels faring worse than petrol engines, it notes. Details of the "seriously adrift" official figures are revealed in a new report called: "MPG claims - Fact or Fiction?." Which? compared the fuel economy figures for more than 100 petrol and diesel cars it has tested this year in its lab against the official figures which car manufacturers cite and which are based on official EU-regulated tests. This revealed that the model "furthest adrift" of official claims was the 1.3-litre diesel Vauxhall Corsa EcoFlex super-mini. Cash-strapped drivers can end up nearly £350 a year worse off than they expect to be The claim was that it achieved 85.6mpg but Which? testers managed only 65.7mpg - a difference of 19.9 miles to the gallon. That difference means the car will cost its owner £269 more to run each year than the official figures would suggest. This was followed by the worst performing petrol car - the Peugeot 208 1.0 litre - with a claimed fuel efficiency of 65.7mpg as against the Which? testers" result of 52.3mpg. This is a difference of 13.4 miles to the gallon which will cost the driver £281 more than expected in fuel bills each year. Third place is the popular Ford B-Max family multi-purpose vehicle 1.6 litre diesel with an official figure of 70.6mpg compared to the 58.9mpg achieved in tests - a difference of 11.7mpg. That will leave drivers £215 a year worse off. The biggest unexpected extra bill overall is faced by drivers of the Kia Sorento 4X4 at £349. Although the gap is just 8.7mpg (the official 47.9mph compared to the tested 39.2mpg) the cash cost bill is larger because this is a less fuel-efficient vehicle overall, so the effect of the difference is amplified. And there's a shock for drivers who choose "green" hybrid car. The Volvo V60 diesel-plug-in hybrid missed the claimed figures by 74mpg, having been tested at 80.7mpg as agains the claimed 155.2mpg) and almost doubling the annual fuel cost. But Which? did concede: "However, it's still the most fuel efficient car we've ever tested. Costing less than £1,000 in diesel over 12,000 miles. The Which? report also noted: "The diesels we tested are less capable of matching their mpg claims than the petrol models." The Which? fuel-efficiency test replicate typical driving, including motorways which EU tests do not. Which? car expert Rob hull said the official EU-regulated test was unrealistic and that most manufacturers flagged this up with "caveats" in the small print. He said: "We know these figures aren't achievable, and small print like this on websites shows that manufacturers know it too and want to cover their backs. The current test doesn't reflect real driving situations. Most of us drive on motorways although there's no part of the official EU test that covers this. And the cars are driven in their most efficient driving mode, which often doesn't deliver acceptable everyday performance. Our test does take these points into account. Car-makers said they had by law to follow EU rules for standardised fuel-economy tests to ensure a "level playing field." And though discussions are under way for new and more "realistic" tests, these were still some years away.