The new, 1981-model American-built small cars continued to sell well in mid-October, but not well enough to prevent an overall drop in sales for the five U.S. car producers.

Chrysler Corp. reported sales of 20,091 during the second 10-day period in October, down from 27,860 in the first 10 days of October, when the introduction of the new Dodge and Plymouth K cars filled dealers' showrooms.

Ford Motor Co. said it sold 40,249 cars in the mid-October period, down sharply from the 61,029 sold in the first 10 days of the month. The drop in Ford sales came despite continuing strong demand for the Ford Escort and Mercury Lynx subcompacts, Ford's new front-wheel-drive model.

Ford dealers have sold 15,677 of the Escort/Lynx models during the first 20 days of October, and the company said it isn't able to build the cars quickly enough to satisfy potential customers.

Despite the lack of a new model fall, General Motors Corp.'s sales increased from 127,573 in first 10 days to 135,700 in the mid-October period.

Analysts said both trends were normal. Ford and Chrysler had exceptionally strong sales in the first part of October, thanks to the interest in its new models. The slowdown in mid-month is not a surprise, company officials said. GM's gradual increase during the month is also customary when a company has no new models to introduce.

AMC, which like foreign auto producers reports sales only at the end of the month, sold an estimated 3,700 cars in the mid-October period, while Volkswagen of America, Inc. sold 2,927 cars made in its New Staunton, Pa., plant.

Together, the five companies sold 202,666 cars in mid-October, for a seasonally adjusted annual rate of 7.2 million cars -- an improvement over the deep sales slump since March, but well under the performance the industry needs to recover from its 1980 battering, industry analysts said.

A Ford spokesman noted that sales for the five domestic car companies during the first 20 days of October were down a little more that 3 percent from the same period a year ago. By coparison, the companies' sales from January to September this year were 23 percent below the comparable period a year ago.

In the year-to-year comparisons, Ford was the biggest loser. Its mid-October sales this year were 39 percent below those of the year before. GM was down 4.5 percent; Chrysler, 7.6 percent.

The comparisons with last year are misleading, company officials said, explaining that this year the new models were introduced early in the month, boosting sales figures for that period, while last year the new models were introduced in mid-month.

Chrysler said the demand for its new, front-wheel-drive K cars, the Dodge Aries and Plymouth Reliant, continued to be strong. Dealer orders for the two models has reached 85,000 excluding fleet orders.

Both Chrysler and Ford are running two plants on overtime to turn out the new models.In Ford's case, its Escort/Lynx output is limited by engine production and delays in solving vibration problems in automatic transmissions which did not start going into the new cars until two weeks ago.

Chrysler said it sold 3,550 K cars in the mid-October period, bringing sales for the first 20 days to 13,767, the fastest start ever for a Chrysler product.

Chrysler officials were cheered by a good showing by some of the company's larger cars. Sales of the Cordoba and Mirada models increased from the first 10 days to the second, this month.