What kind of a load hp wise does it put on the engine and possible effects on mpg?

I like my mpg but... I think I could get used to 31 degree vent temps rather quickly.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)