Fears about the sliding value of property prices in the United States and the surging petrol price hit demand for new cars across America in one of the worst months for carmakers in two years.

General Motors and Ford said that US car sales had fallen by almost a fifth in July. Last month, the price of petrol hit a near-record of $3 a gallon.

Last night, the Japanese carmakers Toyota and Honda also reported a plunge in American car sales for July of at least 7 per cent, while Daimler-Chrysler’s new car sales dropped by more than 9 per cent.

The five car companies – whose businesses make up 75 per cent of American car sales – have not each reported a slide in sales in the same month for two years.

The grim sales numbers were published as the key US oil futures contract for September delivery hit its highest level since 1983 at $78.77 a barrel.

Shares in Ford, which makes the market-leading F series pickup truck – slipped 1 per cent to $8.40 while General Motors stock inched higher to $32.58.

General Motors estimated that car sales across the car manufacturing business in the United States as a whole had fallen 10 per cent in July and said that its full-year sales target was now at risk.

It is not known whether such a slowdown in sales will trigger a cut in car production and further price reductions, lowering margins.

While the market had expected a fall in demand for July, the slide was far steeper than anticipated. Carmakers try to clear their car inventories during the summer and car sales are perceived as a key indicator of consumer spending in America.

The market for pickup trucks, which accounts for about 13 per cent of overall car sales in America, has become the most fiercely contested part of the nation’s motor market, as manufacturers sacrifice margins with expensive sales offers in order to protect market share.

In July, General Motors, Ford, Toyota and Chrysler were all offering interest-free financing and cash rebates to bolster such sales, which are closely tied to construction spending and housebuilding. The incentives are hitting US carmakers as they try to stimulate spending. During July, the average new car incentive cost US carmakers $2,524, up 4 per cent on the same period the year before.

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