There are a lot of "state the obvious" reports out there. Some people just don't think. Not so long ago, driving instructors used to have these monstrosities positioned horizontaly across the roofs of thier cars, until they realised a sticker on the side creates far less drag!
As a Pizza delivery executive, I was instructed to strap a roughly 4' x 2" x 2" giant telephone to the roof of my car.
After just a few hundred yards of anchor like drag I dumped it back at the shop and told him if he wanted to pay me £5 rather than 50p mileage per delivery (it was 13 years ago), I would be happy to reattach it.