Motorists are set to flock to car showrooms next month following the introduction of the new “58” number-plate – but they could fork out nearly £170m unnecessarily by opting for expensive dealer finance packages, research shows.

Almost 407,000 new cars are expected to be sold next month after the new plate is introduced at the start of September (down 3 per cent on last year).

Half of those sold during September 2007 were bought with dealer finance, and if the trend continues this year motorists could waste a collective £168m, price comparison website uSwitch.com says.

The average rate of interest on a car finance deal is 10.17 per cent - 2.77 per cent higher than the cheapest unsecured personal loan, currently 7.4 per cent. Consumers could save £826 each in interest payments over three years, if they go for the cheaper option.

With some dealer finance rates even higher, the potential savings could be greater.

For example, a motorist looking to purchase one of the UK’s top five selling cars, the Vauxhall Astra TwinTop Air, at a cost of £18,215 with a 10 per cent deposit, could have to fork out 11.4 per cent through Vauxhall’s car finance deal. That would cost £2,871.91 in interest over three years.

If the same purchase was made with an unsecured personal loan from Asda at a typical rate of 7.4 per cent, the total interest paid would be just £1,871.83 – more than £1,000 less.

Simeon Linstead, head of personal finance at uSwitch.com, said: “Buying a brand new car is a big expense which can be seriously inflated if the financial arrangements are not researched thoroughly.”

Other low-cost unsecured personal loan rates include 7.6 per cent with Moneyback Bank, and 7.7 per cent with the AA, Alliance & Leicester, Sainsbury’s and the Bank of Scotland, if taken out via the Internet.