When Elon Musk, who loudly disdains the traditional auto industry, makes his first public appearance in Detroit in two years on Tuesday, it will be easy to see how much has changed since then.

Mr. Musk’s Tesla Motors Inc. is worth six times more in stock-market value. He is pushing hard to sell 500,000 vehicles a year by 2020, up from 90 a day in the third quarter. And giant auto makers are on a collision course with Tesla like never before, with General Motors Co. showing off a new electric car at the Detroit auto show on...