The economy comparison test that R&T did a few months ago (Prius, Fiesta, Golf TDI) included a back to back 30-mile freeway run, once with A/C off (windows up), and once with A/C running. I don't cave the article in front of me now, but I recall the A/C penalty being ~2 MPG for the Golf. They did not do a windows down test.
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