US stocks managed some small gains Tuesday, but not enough to make up for Monday’s big losses. Stocks spent most of the day alternating between small gains and losses and turned positive in the last hour. That came a day after a plunge in China’s main index set off a bout of selling in global markets. Despite increased tension in the Middle East, energy prices continued to tumble because demand appears weak and global stockpiles are large. US crude fell 2.1 percent to $35.97 a barrel in New York. Brent fell 2.1 percent to $36.42 in London. Drilling companies had the biggest losses: 6.3 percent for Ensco and 4.8 percent at Diamond Offshore. US automakers said last month was their best December ever, with 1.6 million cars and trucks sold, and 2015 was the biggest sales year to date. But shares of General Motors and Ford slumped 2.6 percent and 1.8 percent, respectively; their monthly sales fell short of analysts’ projections. Gun makers continued to trade higher; Smith & Wesson rose 11.1 percent, and Sturm Ruger gained 6.8 percent. Fitbit fell 18.3 percent to a new low; investors were not impressed with Blaze, its newest fitness tracker.