Use this combo to stay warm and comfortable down to 0 degrees in your car. The Coleman sleeping bag is rated at 0 degrees, but that's the survival rating. To make it the comfort rating add the Coleman liner and one of those $9 red Walmart sleeping bags.
https://www.amazon.com/dp/B0009PUSO4/ref=cm_sw_r_apan_glt_fabc_3PX3GVYJBHEH5Z94PV3W
https://www.amazon.com/dp/B084SV4CXF/ref=cm_sw_r_apan_glt_fabc_PAJG3NBZKBE79WVMK37K