Sales of domestic cars fell 21.3 percent in early February, compared with the same period last year, but the Big Three automakers said they were optimistic anyway.

The sales picture was a cross-current of conflicts arising from incentives offered by the manufacturers.

The five companies delivered 146,947 cars compared with 210,068 in the first 9 days of February 1979. Big Three sales were off 22 percent. Ward's Automotive news speculated that auto backlogs totaled 1,598 million, a 71-day supply.

The percentage changes are based on sales per day because there was one more selling day in last year's period.

Chrysler Corp. sales fell least among the Big Three -- 11 percent. Jerry Pyle, vice president for U.S. automotive sales, said the relatively good showing was the result of the comapny's 30-day, money-back guarantee beginning to "take hold.

Chrysler's share of the market for U.S. models jumped to 11.4 percent from 9.2 percent for January as a whole. It was the company's strongest opening sales period for any month since October, when the company got 12 percent of the domestic market. Chrysler also announced it has been offering buyer rebates since Jan. 21.

It was not clear why the rebates, which give buyers of some leftover 1979 models $200 to $700, were not announced when they began. When asked for an explanation, a spokesman who requested anonymity said, "We just decided to work on it ourselves."

It is the first time since 1975 that all Big Three companies have offered factory rebates at the same time. General Motors Corp. began a $500 offer on some 1979 models on Feb. 1 and Ford Motor Co. later began a $300 or $500 offer on two car lines from both the 1979 and 1980 model years, with a separate $400 rebate on a third line in California only.

Ford sales nosedived 43 percent, which the company attributed to the introduction of the rebates two days before the end of the period. "Every car sold on the ninth is going to become a car sold on the 11th because the dealers held back the cards," said a representative who asked that his name not be used.

GM, which waited until the end of the period to announce its rebates, said sales were off 15 percent. Robert D. Burger, vice president in charge of the marketing staff, said sales were "very encouraging" since the last period of January was a record.

American Motors Corp. sales once an estimated 37 percent, thanks to the continuing demand for small cars. The estimate is necessary because AMC reports only at the end of the month. Volkswagen of America, still feeling the effects of a three-week strike that forced the closing of its assembly plant, said sales fell 23 percent.

In other auto industry developments yesterday:

Importers captured a record slice of the truck market in January, according to Ward's Import sales soared 80 percent in January from 26,081 to 47,016 trucks. Sales of domestic producers fell 30 percent from 277,342 to 188,835.

Of the total truck market, the importers got 20 percent in January compared with 8.6 percent in the same month last year.

Honda Motor Co. said it will market a new fuel-saving car in Japan Saturday. The new car, known as Quint, is to be offered in three types, powered by a new model, high-power, low fuel consumption engine.