Originally Posted by
toThinkistoBe
If you are talking about internal combustion engines (You said mpg so I'm assuming. If it was a typo, just ignore me.
) having the heater on actually makes the engine run more efficiently since the heat is taken from the engine.
I think it's a given that using A/C will kill the battery fast on an electric car. If this is the same with the heater, you'll never want to drive them in the dead of winter.
I rode in an auto in the winter without the heater and it's horrible.