Here's what I can tell you - recently we were hitting temps in the low -30's here with windchill factors well into the -40's.
My father-in-law lives out of the city and broke down on the highway one night. We headed out to help him as soon as we got his call. Bought some coffee's on the way out at a Tim Hortons and arrived within 30 minutes to where he was stopped.
I tell you, in the open with that wind.. he was damn near frozen. A couple candles may have at the very least helped keep the chill off inside that vehicle until we showed up.
Since then I have rechecked our own vehicle kit and added a couple more candles just in case.