General Motors Chairman Roger B. Smith has given up on the idea that a sharp drop in interest rates will automatically trigger a sustained recovery in U.S. auto sales.

"There is no magic point at which everybody runs in and buys. You can't draw a line and say: "That's it," Smith said today in an interview.

Earlier, auto industry officials had argued that once interest rates dropped to 12 percent, sales would begin to take off.

Smith said that short-term fixes, such as GM's new 10.9 percent financing on many of its 1982 models, may help reduce bloated inventory, but that rate is not a magic number that can produce steady sales in an era of lagging consumer confidence.

He said GM's newest financing plan "is doing a good job of sweeping up everybody on the buying curve" between the average 17 percent GM was charging just a few weeks ago and the 10.9 percent rate GM is now charging under the plan it started Nov. 1.

Continued concern about unemployment is keeping many buyers out of the market, despite relatively favorable financing rates, Smith said.

"What we're doing now is trying to get the market cranked up and going," he said. "Back in a couple of other downturns, we had pulled the market ahead a little and had gotten it started faster by doing things like this.

"But if a guy is going to go out there and buy a car, he's going to say, 'Boy, it's going to take three years for me to pay for that. Am I going to have a job for three years? I don't want to go out there and buy a car and default on my debt.' " Smith said.

Under current conditions, U.S. automakers are using interest rates and other incentives to "steal" sales from the future and from one another, Smith said.

Ford is offering 10.75 percent financing on its leftover l982 models. Chrysler is giving "dealer incentives" -- savings that can be passed on to customers -- ranging from $300 to $600 on most of its leftovers. All of the big three manufacturers admit that their strategy -- which some auto industry analysts regard as an act of desperation -- is to clear out showrooms to make space for l983 cars and trucks.

According to the trade journal Automotive News, GM, Ford and Chrysler had inventories exceeding 60 days' supply as of Oct. 1.

GM had enough cars to last 69 days. Ford had 67 days' supply and Chrysler had 77 days' supply. A 60-day supply is considered normal in the industry.

Smith said many things are falling into place that could yield steady growth in auto sales by the spring of l983. He credits the Reagan administration for bringing about the changes.

"I said inflation had to come down, and, wow! it really did come down. I said interest rates had to come down, and they've come down. I also said the stock market and the bond market had to go up, and, boy, they sure have gone up."

Smith said that, "I never could figure out why the president hasn't gotten more credit for bringing down inflation. I think his administration did a fantastic job of doing that."