American automobile manufacturers reported today that 1978 was the best year since 1973 and the third best year on record, with sales of 9.3 million domesticmade cars.

Although sales were on a decline during the last weeks of December, car volume for the month rose 2.7 percent from the 1977 period with 638,192 cars delivered.

Counting truck sales -- which have been soaring -- and imports, total motor vehicle sales last year were at least 15.4 million, up 3.6 percent from the previous record in 1977.

The head of the Cheverolet division of General Motors Corp. boasted of an all-time record for combined car and truck sales in 1978. GM Vice President and Chevrolet General Manager Robert Lund criticized recession-predicting economists as well as pessimists in the news media and forecast still another record for Chevy vehicles this year.

What Lund did not say, however, was that Chevrolet was the only GM division that did not sell a record number of cars during 1978.

GM's Oldsmobile, Buick, Pontiac, Cadillac and GMC Truck and Coach divisions all set record sales in 1978, making the year the first time GM has sold more than 7 million vehicles inside the country. GM marketing vice president Robert Burger said this development is part of the reason his firm is "confident" 1979 auto and truck sales will be a record 15.5 million units.

GM delivered 5.4 million cars in 1978, up 4.6 percent from the previous year, and increased it share of the domestic market to 58 percent from 56.6 percent.

Ford Motor Co. delivered 2.58 million cars, up 1.5 percent; Chrysler Corp. sales fell 5.7 percent to 1.15 million; and American Motors Corp. auto sales declined 7.4 percent to 170,739.

Chevrolet's Lund spoke today at a champagne breakfast in Detroit's Renaissance Center and his message appeared to be that GM is by no means going to satisfy itself with 58 percent of domestic sales. That means trouble for Ford, Chrysler, AMC and importers.

Two important items were not mentioned at the GM breakfast. First, there is a growing consensus in this city that GM will be the United Auto Workers' target when the union's contract expires in mid-September and even a quickie strike would knock any sales projections into a cocked hat. and second, in early spring GM will start building and selling its small, front wheel drive replacement automobile for the compact Nova and its counterparts from GM's three other divisions.

Volume for that totally new automobile, tentatively named the Chevrolet Citation, is targeted at an annual rate of 450,000 although suppliers in this city are talking about an annual rate of 800,000 for all versions of the car, including those of Buick, Olds and Pontiac.

In other developments today:

Ford boosted prices on 1979 model cars an average of $124 or 1.7 percent, completing the third round of price hikes by the major auto makers since the model year began last fall. Price increases range from $35 on Price increases range from $35 on Pintos and $260 for Lincoln Continental Mark V.

Chrysler cancelled plans for a $160 million plant in Richmond, Ind., which was to have produced automatic transaxles for new front-wheel cars in the 1981 model year.