Shares in Fiat Chrysler Automobiles tumbled in Milan and New York after two U.S. dealerships filed a lawsuit accusing the carmaker of inflating U.S. car sales. The suit, filed by dealers under the Illinois-based Napleton Automotive Group, charges that the incentive programs from the Italian company's U.S. unit, FCA US, rewarded dealerships that falsely reported higher car sales, inflating the auto giant's results. The lawsuit cast a cloud over FCA US's announcement on Jan. 5 that it had sold a record 2.2 million vehicles last year, a 7 percent gain from 2014 on the back of a sharp surge in December. The U.S. unit is Fiat Chrysler's most profitable arm and has boasted of 69 straight months of year-over-year sales gains. Fiat Chrysler strongly rejected the accusations and vowed to defend itself "vigorously" in the case.