Aerodynamics have a huge effect. My Scion xD gets about 36 MPG on average without towing. With a bicycle on the roof, that drops to about 31 MPG. I got 32 MPG towing a 4x8
Uhaul trailer that weighed about 1,500 pounds. Believe me, my bicycle weighs a lot less than that!
I got 24 MPG towing the
Scamp up to the Poconos
rally last weekend.... That was with two bikes on the roof.