U.S. automakers sold 17.2 million vehicles in 2017 , nearly 320,000 fewer than 2016, snapping a streak of seven straight years of sales gains. But they did benefit from higher prices for some vehicles, and more demand for higher performance vehicles like SUVs and electric cars. A bullish outlook for 2018 could be dented, however, if the latest tax reforms hurt middle-class spending, or if the federal probe into Russia’s suspected meddling in the 2016 U.S. Presidential election destabilizes the Trump administration, experts say. Meanwhile, automakers have differing preferences in choosing the right technology standards for autonomous vehicles, and changes are likely on the way for fuel economy regulations. Even as auto sales were down last year, “it was a strong year,” said Wharton management professor John Paul MacDuffie , who is also director of the school’s Program on Vehicle and Mobility Innovation at the Mack Institute for Innovation Management .…