Thorn;
so what are you driving? Just curious.
I drive a Prius. A large share of its gas mileage comes from its aerodynamics. So, throwing a bike rack on it, even on its rear end, disturbs its slipstream. I used to blame the bad mileage with the rack on my DH not knowing how to drive the Prius, but, alas, I also got the low mileage with the rack so he's off the hook.

Of course, the severe coldsnap we had a few weeks back also whacked the mileage. When it was hovering around 0F, the electric motor rarely kicked in (it felt so odd sitting a a stop light and having the engine run!). During that week, I was barely eek-ing out 40mpg (winter normal mileage is mid-50's; I routinely hit 60 for the tank in summer). Still, 40mpg with minimal electric boost for a car that size is pretty good. I suspect that is attributed to its aerodynamics more than anything else.

But, I'm a software engineer and only dabble in things mechanical--enough to stick my foot in my mouth.