President Donald Trump has accused America's biggest oil companies of ripping off drivers at the gas pump - ordering the Justice Department to investigate after crude prices plunged but retail prices did not fall as quickly as he wanted.In a furious post sent after midnight on Wednesday, Trump said oil companies were failing to pass on the full benefit of sharply cheaper crude to ordinary Americans.'The big Oil Companies are not dropping their price at the pump commensurate with the sharply lower prices they are paying for Oil,' Trump wrote on Truth Social.'Those prices are dropping like a rock! In...