American Motors Corp. two-wheel drive cars will average 23 miles per gallon in the new model year, the company said yesterday. It's four-wheel-drive cars and Jeeps will average 19.1 mpg.

The No. 4 automaker is the third U.S.-owned manufacturer to announce its fleet average for 1981 models. Earlier, Chrysler Corp. said its cars would hit 25.5 mpgh and General Motors Corp. said its cars would make 22.9 mpg.

AMC unveiled on Wednesday the newest addition to its import line, calling it the first truly cooperative result of its partnership with French auto giant Renault.

But AMC officials denied that Renault is about to exercise its option in that partnership agreement to boost its holdings of stock in the U.S. automaker from 5 percent to 22.7 percent.

Speaking at the news media "preview" of the company's front-wheel-drive Renault 18i, AMC President Paul Tippett said the matter of Renault increasing its AMC holdings has not been discussed.

The company's U.S.-made cars and Jeep products -- including a new, scaled-down version of its four-wheel-drive Eagle -- was being show yesterday. The new lineup will be in dealer showrooms Sept. 25.

AMC's linkup with Renault has company officials in a decidely upbeat mood, despite the slump that has gripped the U.S. auto industry for more than a year.

Tippett said the company hopes to sell about 161,000 cars, 100,000 Jeeps and 56,000 Renault products -- both the new 18i and the Le Car -- in 1981.

Chrysler Corp., meanwhile, has boosted the sticker prices on its new K-car compacts.

Dealers were told Wednesday the introductory base price would be $127 more than planned, and options would cost less. Chrysler had told dealers the cheapest K-car would list for $6,065. Wednesday, dealers were told that revised K-car prices would put the cheapest at $6,192.

An industry publication, meanwhile, reported that U.S. auto production in September will plummet to its lowest level since 1970 with all five domestic carmarkers forecasting production declines.