The credit crunch and continuing high petrol prices, plus rising unemployment and tightening credit standards have conspired to force US car sales sharply lower last month.

According to figures compiled by the consultancy AutoData, sales of new cars and light trucks dropped 27% in the US last month. That was more than double the 11.2% drop in sales in August.

From other figures, September looked like the worst month for car makers since January 1991 and it is possible that less than a million cars were sold in the country for the first time in a long time.

Sales from the major carmakers fell sharply in September, as tighter credit for buyers added to pressures already forcing buyers to stay away from showrooms.

Figures from some analysts reckon that dealer traffic fell 50% after the middle of the month as the financial crisis erupted anew and it’s not surprising that General Motors largest Chevrolet dealer in the US went into bankruptcy protection on Monday as a result.

The sales declines were broad-based, with Japanese automakers reporting the same kind of double-digit declines that have whacked Ford, GM and Chrysler. Many prospective buyers have been unable to get the credit they needed to buy a car and a growing number of dealers have had their own credit cut off or curtailed, causing widespread failures. And comments by car company executives suggest they don’t think they’ve seen the bottom yet.

Sales of cars and light trucks fell 35% from August 2007 at Ford, 32% at Toyota and 24% at Honda. GM’s were down 16% (which was taken as good news) Nissan reported a 37% drop, Chrysler a 33% fall and Hyundai, 25%.

Kia’s sales fell 28%, Ford associate, Mazda, saw its sales slump a massive 36% on August 2007.

Among smaller brands, Fuji’s Subaru had a good month; sales were down just 12% but there was only misery for Mitsubishi with a 39% drop, and even worse for Suzuki with a 47% plunge.

Chrysler, which includesDodge and Jeep brands, saw sales of its light trucks drop 34% and car sales lost 29%.

Toyota’s September drop was the sharpest percentage drop in US sales for the Japanese giant in 21 years. It cut output in Japan by more than 17% last month as well, while Honda and Nissan also cut production in their home markets.

It was Honda’s worst monthly sales performance since 1981, and a sign that the good fuel economy it boasts of for its models, is no longer enough to buck broad industry declines.