trailer hitch question

[TXSUT]

Your best bet is to take some measurements based on the height of your trailer's tongue, but the standard answer around here has always been either 6" or 8" of drop (not necessarily to clear the spare tire, but to ensure proper ride-height of the trailer). That's going to depend on the aforementioned trailer tongue, tongue weight, and whether or not you have rear air bags.

There's several threads on the topic, so do some searching.