Baron OSX, I don’t think there’s a quick easy answer.
Maybe try one of these approaches:
1. If you already have the talent recorded but not in a car, I would separately record some inside-the-car background sound (ambiance, engine noise, traffic noise, etc) then layer it under the already recorded dialog. In short, fake it.
2. Another approach might be to record someone speaking in a car. Use that recording as a reference point. Then apply FCPX audio effects to your actual clip and experiment, with the goal being to replicate the reference sound. Trial-and-error.
The thing is, different cars have different sounds. It depends on the size of the interior, the amount of plastic that would generate echos, the amount of carpeting that would dampen the sound, the kind of engine, the road surface, etc.
Ultimately the answer depends on what you have for audio, and what you’re trying to achieve, and how accurate it has to be.
Hope this gives you some ideas.