You would be very surprised at the number of things done already incorporated into our vehicles that will and do improve the aerodynamic aspect of today's cars. There was a time when that was not true but today with fuel being what it is fuel mileage improvements will help to sell cars/trucks. If some of these hype after market items really made much of a difference the manufacturers would already be doing at least some of it.
TRUE STORY. Several years ago I found out that the school district, at the advice of a new assistant superintendent installed on (70 school buses) around the fuel line a set of 3- magnets at a cost $200 a set. It was supposed to improve fuel mileage. If it really worked, which it does not, why wouldn't the manufacturer run the fuel line next to or around the alternator, which has a magnetic field all the time??? Because it does not work. Oh, by the way they never bother to call me, the local automotive instructor who would have advised them not to spent the $14,000 plus installation. I probably would not have charged them more than a couple of grand for the advice.