Tetley used to fart real bad when he was a tiny puppy but we changed his food and now he just gives one great big belch after his dinner. Sounds really funny! It's like clockwork too - he'll finish eating, walk over to where me and hubby are sitting on the couch and we'll look at each other and say 'here it comes!'. Sure enough - "buuuuuuuuuuuuuurp!". I just take it to mean he really enjoyed his dinner.
Don't know what you can do about the car farts though! He's probably just nervous.