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Don't forget assuming you plug it in 8 hours a day for 20 days a month at 400 watts this consumes 160 x 400 / 1000 = 16 x 4 = 64kwh's, so assuming a cost of 11 cents per kwh your electric bill will cost $7.04 extra per month in this particular scenario.

When you find out your actual mpg increase you will want to calculate how much actual fuel it saves, considering that the money spent on the electric would have bought 2 gallons of fuel it is hard to see at this point if this really saves money because heating an engine is one of the most inefficient ways of electricity usage.

Now I do see you're in a cold climate so if the temperatures around your car are in the single digits Fahrenheit this might affect the car to where it would not start unless it was heated then a block heater is the answer of choice, which is to say I have never heard of anyone using such a device unless it was absolutely crucial, such as the car would NOT start otherwise. And yes, there is such a thing as the kind of cold that will affect a car in such a manner, but if it is kept inside and it starts fine unheated then this is as likely just so much more phony baloney and snake oil.

It is one thing to get better economy, it is quite another to push a car up a hill so one can coast down it to get better mpg and then go spend $5 more on a lunch when it would've cost 50 cents or a dollar just to drive up it, but hey, I get unlimited mpg.

Should your calculations differ from mine on the electric bill and the mpg increase make this truly worthwhile I am certainly more than willing to stand corrected. Because I'm not knocking experimentation per se but to ignore the entire cost of the other side of the equation, that frustrates more than not worrying about it in the first place.