I absolutely agree with what Jay says, but it's also worth noting that using the sound from the interior of the vehicle even during lengthy external shots is a little used but effective technique. Most audiences will naturally connect the sound of the interior with the external shot of the car, but if you use this technique carefully it can really force the audience to listen as (depending on the shot) they may not be able to see the people in the car. One film which uses this extensively is A taste of cherry which has long sections of interior car dialogue with wide angle car shots. Unconventional, but it really works. Granted, this is not a technique that's going to be adopted for prime time tv, but is interesting none the less.