Albert Barnes Commentary 2 Kings 9:20

"And the watchman told, saying, He came even unto them, and cometh not back: and the driving is like the driving of Jehu the son of Nimshi; for he driveth furiously." — 2 Kings 9:20 (ASV)

The driving ... furiously - The word translated “driving” means “leading” or “conducting” a group. The watchman observed that the “company” (or multitude) was led forward frantically and associated this strange procedure with Jehu's known character. It is curious that some versions, as well as Josephus, give an opposite meaning: “he drives quietly.”

Jehu was, properly speaking, the “grandson” of Nimshi, who was probably a more famous person than Jehoshaphat (2 Kings 9:2).