RISING fuel costs are expected to accelerate the switch from large cars to smaller, more fuel-efficient vehicles and diesel power as cars with the new 55 registration plate go on sale today.

Against a fall in overall new car sales, supermini vehicles and diesel engines have continued to buck the trend as motorists seek more miles for their money. According to the Society of Motor Manufacturers and Traders, diesel sales have almost trebled since 1999 as engines and range of models have both improved. Diesel engines have up to 30 per cent better fuel economy compared with petrol, the society says.

Christopher Macgowan, chief executive, said: “Fuel costs have risen steadily in the last decade and now petrol has breached the £4 a gallon mark for the first time. So, while the mantra for many September customers will be big and beautiful regardless of fuel costs, for many more, bang for your buck will be the priority.”

Ten years ago superminis, such as the Ford Fiesta and Vauxhall Corsa accounted for 27 per cent of the market. Last year they took a 32.7 per cent share. The share of diesel engines rose from 20.8 per cent to 32.5 per cent.

Carmakers hope that the new registration plate will boost the market. While new car sales hit record highs last year, the market from January to July fell 5.9 per cent.

Although superminis and diesel cars have greater fuel economy than standard vehicles, the overall fuel efficiency of all vehicles has improved.

New cars did an average of 37.8 miles to the gallon last year compared with 34.5 miles to the gallon in 1998, the society says.

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