By three o'clock in the morning Dave and Romana are asleep and Jack is in a car with a next-door neighbor of the mad rooming house in the city.
It seems Jack claimed that Cody and Dave were the two greatest drivers in the world, and this kid ("a Bohemian but also a laborer, a housepainter") jumped in and said he was the greatest driver in the world, because he "used to drive the getaway car."
So he decided to demonstrate to Jack, which is why they're zooming all over the city at three in the morning.
Jack barely remembers his name – "Bruce something or other" – but he decides that, yes, this kid is in fact the greatest driver in the world. Jack never saw him again after that night.