CHICAGO—U.S. auto sales were set to smash records in 2015 as easy credit, hot new vehicles and strong consumer confidence lured buyers to showrooms.

General Motors, Ford, Fiat Chrysler and Toyota reported strong December sales on Tuesday and forecast the good times would keep rolling in 2016.

"The U.S. economy continues to expand and the most important factors that drive demand for new vehicles are in place, so we expect to see a second consecutive year of record industry sales in 2016," said Mustafa Mohatarem, GM's chief economist.

"The single most important pieces are the ongoing gains in employment and the growth in personal income. When you add in lower energy prices, it's easy to see why consumer spending is strong."

GM forecast that total industry sales for 2015 would hit 17.5 million vehicles once all carmakers report their results later Tuesday.

That would beat the previous record of 17.4 million set in 2000 and be 6% higher than the 16.4 million vehicles sold in 2014, according to WardsAuto.

"It's truly remarkable that the auto industry is finishing off its best year ever just six years after the depths of the Great Recession," said Jessica Caldwell, an analyst with the automotive website Edmunds.com.

While easy credit terms and low gas prices are making it easier for consumers to buy or lease new cars, Caldwell said the attractiveness of product offerings is sealing the deal.

"If you're buying a new car today, you're getting a safer, more fuel-efficient and more technologically packed vehicle than ever before," she said in a statement. "Automakers are doing a great job giving the people what they want in a new car."