DETROIT Trucks outsold cars by the highest margin in nearly five years in October, a sign the economy might be starting to improve.

These trucks aren’t the tractor-trailers that haul freight. They’re pickups, SUVs, minivans and smaller SUVs. The category — known as light-duty trucks — made up 54 percent of all new U.S. vehicle sales last month, compared with 46 percent for cars, according to industry tracker J.D. Power and Associates. It’s the biggest difference since December 2005, when trucks accounted for 56 percent of sales.

Strong truck sales make economists giddy because they mean people are willing to spend money again. Small business owners feel comfortable enough to buy a new pickup truck or delivery van for their company; and regular folks are confident enough in their jobs and finances to take on beefy SUV payments.

Here are the trends behind the stronger truck sales:

• People are spending more.

• Gasoline is affordable. Truck sales have been sensitive to high gas prices in the past, falling dramatically when gas topped $3 and $4 a gallon in 2008 and drivers were spending $100 to fill their tanks.

While some buyers may need to buy a truck, they don’t necessarily have to buy a new one. And given the increasing durability of cars and trucks, drivers can hang onto their vehicles for longer.