Mostly free heat in modern cars. We should be careful not to give the OP an excuse to freeze his family in the winter by thinking it will save gas, though. A gust of wind could cause more of a MPG fluctuation than using the heater.
My 59 VW had the gas heater which would use gas from the tank to run it. But you’d never have to use it full time or while on the highway. The normal vent heaters worked good enough for that and the vent was right by your feet so kept your feet warm.
I had a 61 Corvair and didn’t have a gas heater with that one. Unsure if and when they were available, but again the exhaust heaters worked fine in Minnesota except for the smell. The wife complained about the smell on her clothes. Never did get that fixed.