what business uses the most gasoline in U.S.?

Answer 1

As far as a business, Walmart does. With their huge fleet of trucks delivering anywhere from 1 to 5 semi's full of merchandise, to each store, do the math.

As far as a the U.S. consumption, the U.S. government takes the cake by far. No need to explain here.

Answer 2

Most corporations with large fleets of vehicles mostly use diesel fuel. I believe it is probably the Postal Service. Their delivery vehicles run on gasoline.