I do have a bike rack on my HCH. It is a hitch mount Yakima that is supposed to be able to carry 4 bikes. I have noticed that with 1 hybrid style bike, going around 60 - 65, I was getting in the upper 40's. Of course this is starting from the upper 40's anyway, but the mileage did drop noticeably with the bike at these speeds. Around town with 2 or 3 bikes I don't notice a drop in mileage.

Billy