General Motors Corp. Chairman Roger B. Smith confirmed today that the new car to be produced jointly by his company and Toyota Motor Corp. Ltd. will be based on a version of the sub-compact Toyota Sprinter now sold exclusively in Japan.

The basic difference between the two cars is that the Sprinter is an in-line engine, rear wheel drive model. The GM-Toyota hybrid will be produced and sold in the United States as a transverse engine, front-wheel-drive car.

Smith's comments in an interview marked the first time that he has offered an on-the-record description of the proposed GM-Toyota car, scheduled to be produced at a former GM assembly plant in Fremont, Calif. GM and Toyota officials hope to begin production as soon as possible in 1984.

Critics of the joint production agreement, signed in Fremont on Feb. 17, say the pact may violate federal antitrust laws, largely because it joins together the world's first and third largest automakers. GM is No. 1 and Toyota is No. 3 in world auto production.

Either GM or Toyota could have produced the new car separately, critics say.

Smith, as he has in the past, denied that charge today. But this time he elaborated, saying that the GM-Toyota combination was serendipitous happenstance, "like seven lemons coming up all in a row on a slot machine."

"This is a one-shot thing, like two ships that pass in the night, and it just happened to hit. It'll probably never happen again," Smith said.

He elaborated: "Toyota had a car, you see, that they couldn't produce in the United States. It is called the Sprinter, that's what it is. Everybody thinks it's a Corolla. But it's not the Corolla.

"We had an empty plant" in Fremont, Smith continued. "And the fact is that they were selling the Corolla here in the United States already. They didn't need to bring the Sprinter in for that, but they could bring it in for this deal.

"Nobody else had a modern, transverse, front-wheel-drive plant, and that's what was needed to make this car. We just spent a hell of a lot of dough remodeling the Fremont plant to produce the transverse, front-wheel-drive cars," Smith said.

GM in 1981 spent more than $100 million to remodel its 20-year-old Fremont facility. The design included the installation of 17 new robots.

The other part of the good luck was provided by Toyota, Smith said. "Nobody else had a car" like the Sprinter "that is engineered and is on its feet," he said. Smith said that GM's "concession to the Americanization" of the Sprinter "is that we're going to build a stamping plant next to the assembly plant" in Fremont.

With the new metal-stamping plant, Smith said, the joint venture partners "can get an American styling look, which I think is going to improve the car considerably."

Smith said that those "combinations of circumstances" leading to the joint venture agreement, which is supposed to last no longer than 12 years, "will never happen again." But he said the lemons came together "at a time when we needed a transverse, front-wheel drive small car."

GM's current arsenal of small cars includes only rear wheel drive T-cars, the Chevrolet Chevette and Pontiac T-1000, which have been produced in the United States for the past 10 years. The company is in need of newer small car models, which it said it could not produce competitively because of a $1,500 to $2,000 cost advantage enjoyed by the Japanese in small-car production.

Smith said that the Americanized Sprinter will be designed to fill that gap. But he denied, as he also has done in the past, that GM has any intention of ending production of the Chevette and Pontiac T-1000.

The Sprinter, as now produced in Japan, has a 1.6 liter four-cylinder engine which can be equipped with an automatic transmission, or 4- or 5-speed manual transmissions. The car has a wheel base of 94.5 inches and a turning diameter, curb to curb, of 30.8 feet. It comes in four-door, two-door and hatchback models. Toyota officials said today that many of those dimensions are likely to change, mostly because of the new engine configuration and drive-wheel arrangement of the Americanized Sprinter. "The American Sprinter will be all new," said one Toyota official, speaking anonymously.

The American car, industry officials say, will be a four-door sedan, retailing at about $6,500. Toyota and GM hope to produce 200,000 Americanized Sprinters each year.