Coming in by car and by air are different. If you come in by car, you can use a FM-T to cross the border as many times as you will during the term of the visa. You also need to go into a bank and pay the $22 fee somewhere along the way.

But if you come in by air, the FM-T is good only one time. You can extend it, but everytime you fly into Mexico, you need a new one, even if you came in by car earlier and have one that has not expired.

My main point is, when you leave the country by car, you should stop at the Migracion office at the border and turn in the visa -- assuming you've paid the $22 -- and have the stamp in your passport canceled. Otherwise, the next time you try to enter the country by car, they will try to collect $5 for every day the uncanceled visa has expired.