In the old days I used to drive over the hill every day with my bike on the roof. My roof rack was a Thule with two side arms, a fairing and an old velo vice in the middle. My little Toyota Corolla Matrix (AWD) used to get around 21-22 mpg. Recently I've been lucky enough to be able to ride from my house and so I was only driving over the hill with an empty rack on and my mileage shot up to 24-25 miles to the gallon. Well since I wasn't really using the rack much and my daughter now fits in one of those easily moveable booster seats so I can put the seats down if I need to get a bike in the car I took the rack off and now I'm getting 28 mpg. Something worth thinking about.
I always wanted a trailer hitch (Thule T2) for this car, in fact I have one, but it's the 2" version for the truck and you can't get a 2" hitch for the Matrix. I contacted Thule about getting just the smaller hitch connector, or the hitch and bit the trays connect to and not a whole T2 so I can put it in a 1&1/4" hitch but they don't sell them. One day I saw a matrix with a 2" hitch, I followed it until it parked and asked the owner where he got it, he looked at me smugly and said, "I made it!" Sigh, he wasn't willing to make one for me. Yup, I could have got a 1.25" to 2" converter but it would have made the rack stick out even more and I think hitch racks have too much torsional force to introduce any more play into the situation.
I wonder if I would have broken even if I'd bought the 1.25" hitch and new T2 and what kind of gas mileage I would have got with that? It sure would be easier to take on and off.