When the sun roof is open, a small "spoiler" pops up anyway.....and I think there is very little drag with open sun roof, since the air stream over the car basically goes up and over....you do hear some turbulence, so there is some drag, which would be lowered by one of those visors, but I don't think you would see any real noticable effect on mileage. (those visors are nice for allowing you to use the sun roof in the rain). If you had a bike or luggage rack on top, that would be a different story....those spoilers they sell for these racks have a big effect in lowering drag...but put a couple of bikes up there, and I think there would be measureable effect on mileage.

If you want to absolutely maximize mileage, avoid fast acceleration, passing, etc., and most important keep highway speeds between 60 and 65.....I can get 30 mpg max like this. If I travel my usual speed of between 70 and 75, my mileage drops to 25.

good luck,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)