N. American Auto Sales Rise 4.8% : Manufacturing: The numbers may understate actual mid-November performance, analysts say.

Sales of North American-built cars and trucks rose 4.8% in mid-November, auto makers reported Tuesday. But some industry analysts said sales probably were better than the numbers indicated.

“They’re lower than I thought, but I haven’t figured out why yet,” said David Healy, an auto analyst with S. G. Warburg & Co. in New York.

“The pattern (each month) has been a poor first 10 days, a reasonable middle and a strong final 10. This time, we’ve got a poor middle 10, but on a 30-day moving basis, we’re still OK,” he said.

The government’s report Tuesday on durable goods purchases for October appeared to support Healy’s assessment. Orders to U.S. factories for big-ticket items such as automobiles shot up 3.9% in October, the biggest gain in 15 months.

Light truck sales continued to record double-digit increases over last year and make up a larger part of the overall market. Auto companies are pleased because they typically make a bigger profit on trucks than on cars.

Sales of light trucks rose 14.4% in the Nov. 11 to 20 period, while sales of passenger cars declined 1.3%.

“I believe that the numbers reported are substantially below actual sales rates,” said Ronald Glantz, an auto analyst with Dean Witter Reynolds Inc. in San Francisco. “Sales are just fine and what we’re going to find is a spurt in the last 10 days as happened in September and October.”

Glantz believes that some dealers intentionally withhold sales from the first and second reporting periods.

“No one can force Crazy Eddie to report sales promptly,” he said. “If there’s a sales contest going on, Crazy Eddie doesn’t want Insane Charlie to know he’s sold 48 cars. And if these guys smell an increase in incentives (late in a month), then they won’t report sales.”

Sales by the U.S. Big Three and Japanese makers with plants in the United States improved over the same period a year ago. Combined Big Three--General Motors Corp., Ford Motor Co. and Chrysler Corp.--car and truck sales were up 4.6%; Japanese maker sales were up 6.6%.

Among the Big Three, Ford had the biggest gain in car sales--up 21%--while Chrysler, which does not report its 10-day sales, had an estimated 34.3% gain in truck sales. The trade journal Ward’s Automotive Reports estimates Chrysler’s sales.