[brucepick]

My view is, since the heat you get from electricity costs less than the heat you get from gasoline, any preheating of the engine should give you benefit. Unless of course the engine never gets a chance to cool down from operating temp.

Remember, when the car needs to heat up the block, it will run rich because that's how the computer is programmed. Much of your warmup fuel will be burning in the exhaust system where it doesn't heat up anything you really care about, never mind giving you efficient motive power.