I didn't notice any change in mileage after I installed my air deflector. I still get about 18-19 mpg(driving 30% highway, at least). On an interstate trip to upstate NY I got about 24-25 mpg, which is the same as before. I do notice that most of the bugs now hit the windshield about halfway up instead of all over. There was one time a big rock from a dump truck hit the deflector when I was on the interstate, and it only put a little scuff mark on it, so it saved the paint. I minor annoyance is that leaves can get caught between the deflector and hood, so I need to spray a hose in there every once in a while. As far as handling, no difference. In high wind situations I do hear a bit more wind noise at highway speeds, though. Here in NJ it's a help in the winter because a good portion of salt spray now hits way up the windshield when highway driving, so I have to use the window wash much less. A cargo carrier has the problem of increased drag in the front and increased vacuum in the rear, so it really affects mileage.