Detroit ? All of Detroit knows all too well that car and truck sales for 2011 will stall if the U.S. economy stops accelerating. But auto sales depend heavily on the ability of consumers to borrow, too.

Most people are not taking stacks of hundred-dollar bills — or writing checks for the full amount — when they buy a new car. So lots of deals didn’t get done when lenders put a vise grip on car loans during the depths of the recession.

But after three years of sharp declines, forecasters see positive signs for the economy and credit.

Jeff Schuster, executive director of forecasting at J.D. Power and Associates in Troy, Mich., said consumers who want to take out auto loans are likely to succeed in 2011, continuing a trend that took off slowly in the summer.

Schuster said J.D. Power expects U.S. car and light truck sales to reach 12.8 million in 2011 — well below historical levels but well above 2009’s 10.4 million. Last year, Americans bought about 11.6 million vehicles.

For some younger buyers, the car market could be particularly inviting in 2011. Schuster noted that many consumers 18 to 35 had been shut out of the new car market during the recession because of their lack of credit history and lack of savings for a bigger down payment.

Younger buyers accounted for 26.5 percent of new vehicle sales in 2006. But that number fell to 20.2 percent in 2009 — and since then has been increasing gradually.

Now, he said, younger consumers could benefit significantly by more available credit — and that’s likely to help boost car sales in 2011.