He said: Translators should make sure that the readers and hearers will know who is speaking here. New International Version and New Century Version indicate clearly that it is Jehu.

See my zeal for the LORD: The Hebrew text has an imperative verb form here, but this may have to be translated by a future tense in many languages; for example, Revised English Bible has “you will see my zeal for the LORD.” The Hebrew word rendered zeal is closely related to “jealousy.” In fact, the word used here is elsewhere translated “envy” (Eccl 4.4; 9.6) and “jealousy” (Num 5.14-15, 18, 25, 29-30; Job 5.2; Pro 6.34). The term suggests a strong and single-minded devotion to a cause. In this case it is the cause of Yahweh. The whole clause may be translated something like “you will see how dedicated I am to Yahweh.”

He had him ride in his chariot: The Masoretic Text has “they had him ride in his chariot,” but few modern versions follow this curious reading. Instead, they follow several ancient versions, as in Revised Standard Version. The meaning of the passage seems to be that Jehu and Jonadab rode together in the chariot belonging to Jehu. Critique Textuelle de l’Ancien Testament gives a {B} rating to the Masoretic Text, noting that evidence from this period indicates that war chariots often had two or three occupants. So the Masoretic Text means that Jehu and an unmentioned warrior had Jonadab ride in the chariot with them.

Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on 1-2 Kings, Volume 2. (UBS Helps for Translators). New York: UBS, 2008. For this and other handbooks for translators see here .