How long is too long to leave it running?

was car camping for a good 10hrs in 10-30s degree weather (might've dropped lower at night, but like I know, I was cozy and warm in the car)...I can say the HSD is good enough to have only used up a little over a gallon of gas by the time I woke up and decided to take off.

I'd take being nice and cozy over cold and freezing or hot and sweaty and for only a little over $2 in 10hrs

 

Generally, if you're stopping at a rest area, you are driving interstate or a controlled access highway, so the ICE is going to fire up regardless when accelerating to speed. Nothing gained by not shutting down, other than retaining A/C (handy when traveling with a pet or the grand-kids!).

Just kidding about the grand-kids .