I have no idea about the gas mileage but have to admit that I always have assumed it does impact it. I have the same bike rack as you. I do drive a lot with my bike - to and from my family in Philly (3 hrs) and Pittsburgh (4 hrs) and a couple times to my family in Canada (8 hrs). I've never worried about this rack the way I did about the one I had before it. With that one, I swear I drove the whole way staring at the bike in my mirror.