But it would be interesting to see the change in efficiency between roof mounted bicycles and hitch mounted bikes. Notwithstanding Foxbiscuits’ remarks (and they are substantially correct!), a roof mounted bike is in a localized area of higher pressure which increases drag, whereas a hitch mounted bike is in a localized area of lower pressure which, though more turbulent, should lower total drag. I certainly can’t predict the outcome, but it would be interesting if some curious soul out there could test it