Italian-American automaker Fiat Chrysler on Thursday unveiled a plan to invest $1 billion in a Michigan assembly plant and offer bonuses to tens of thousands of employees, saying the measures were possible because of US tax reform.

The third-biggest US car maker said the investment would help it to modernize a plant in northern Michigan to "produce the next-generation Ram Heavy Duty truck," shifting production of the vehicle to the US from Mexico by 2020.

To accommodate the increased production volume, FCA will add 2,500 jobs -- a figure that is "above and beyond" the job creations announced a year ago.