As long as your vehicle is big enough for you to lie down fully, both setups have the potential to be equally comfortable. (But if you can't fully recline in your car, a tent will likely be the best option.)
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Answer. The easy answer to this question is that you will never run out of air. You will just exchange the breath that you breathe in with the breath that you breathe out.
Choose a parking spot with maximum airflow around the car to allow exhaust to dissipate. Avoid garages or enclosed spaces; choose open-air lots instead. The idea is for the exhaust to blow away from the car instead of getting pulled into the cabin by the heat or air conditioning system.