My experience with temps is this. Bring a sleeping bag that is rated a little bit warmer than the expect conditions (if it is supposed to be 40 degrees, bring
at least a 30 degree bag). I'm gonna assume that everyone is bringing a tent since this is car camping. A tent will usually add about 10 degrees of warmth, because of wind chill and they hold in heat to some extent. Also, a decent sleeping pad is VERY helpful. A a super thin foam pad (the rolling type) doesn't provide that much warmth. A therma-rest foam/air pad, even the thin one provide a world of extra warmth.
I will be sleeping in a 20 degree down sleeping bag on a thin therma-rest air pad in a 3 season tent. However, im not recommending to go out and buy all this stuff because that's about $500 in camping gear, I'm just giving an example of what i've found works. That set up has suited me well for years in many different tempature ranges. Also, I'd doesn't hurt to bring a space blanket as an emergency item. They add a lot of extra warmth if you need it are very light and pack small, i wouldn't rely on that though because they're a pain in the ass to get back in the little plastic bag they come in. And if push comes to shove, hit up the car heaters :lol: