I have no issues with loading my bike in behind the front seats but I'd like to clear up that space for passengers or cargo on trips.

I know that roof-mounted racks have an effect on fuel economy. What about hitch-mounted racks?

 

Last summer, I went in my Fit (manual trans) for 1100 miles with 4 people in the car plus luggage, AC on most of the time, with two bikes on the hitch mount the whole trip, 75mph cruise control most of the way...average MPG for the trip: 34.5mpg

I've never done the trip without the bikes on the back, so I don't have that info for comparison.

 

I don't think it really affect the mpg at all. I have the big Thule double tray hitch mounted bike rack and I didn't really see much of a difference. But then again, its difficult to make a comparison due to the fact that I usually drive out to the mountains with my bikes.

I would definitely say that it is no where near as bad as having a roof mounted bike rack