Tennier Industries - Modular Intermediate Sleeping Bag

If car camping .....my all time favorite is actually using 2 sleeping bags ....on bottom: a US military, winter, down filled mountain mummy bag, un-zipped ...and on top: a Jansport, extra loft, synthetic down mummy bag, also un-zipped .... with the foot portion tucked into the mountain bags foot portion or "box"..... the 2 bags, opened up, adds additional space, without being all cooped-up .... have used this combo for years, and its super comfy and warm .....even with snow on the ground... outside of tent

I would suggest a military sleep system (USGI MSS) as my sleeping system in a car, in the cold. With all the layers you would be good down to significant cold, especially if you wear dry clean long johns when you go to sleep. They are very affordable for the rating and they are made of a few different layers that can be mixed and matched for warmer nights too.