My Yaris came stock with the antenna embedded inside the rear window. I wonder what the FE improvement is.
Using my new Scangauge in an unscientific test, I noticed a ~10% FE reduction by sticking my arm fully outside the window (50mpg to 46mpg). This was at about 60 mph on cruise control. An antenna is hardly the same as an arm but I can see why it would be bad, being in such a high velocity air stream.
What a coincidence - I was looking at my antenna yesterday while riding past the car on the beater bike ... and wondering why I don't take it off.
Mine's mounted in the same location, but there isn't even a radio on the other end of the cable. I have the original "blank" where a radio should be. (And I've checked: there are 4 speakers installed from the factory. But no radio. Strange. The only option the car came with: side moldings.)
I guess until I get around to putting a radio in, I could take my antenna off too.
This one's in that category of changes where the effect is indetectible, yet logically, it's better without than with.