I asked a similar question a little while back. I can now report my success. I ended up with a custom hitch (from "hitch crafters") and a Hollwood "sport rider" rack: http://hollywoodracks.com/2&3.htm#sportrider I just did a road trip. Prius mileage following a UHaul truck (at a safe distance) 400 miles was 57 mpg, averaging 60-65 mph, in 91F heat, air conditioning on, my personal best. Going the other way (no UHaul), with a person, the bike-rack, and a bike added, averaging 65- 70 mph, in 100-108F heat, air conditioning on, was 50 mpg. I’m happy with that. It's amazing. The bike rack apparently rides well enough in the slip-stream. I've had roof racks for previous cars ... I love how fast the hitch rack comes on and off.
I have a rack on top of mine with the fairing, and have noticed really no loss in mpg...it's probably so minimal, that I don't catch it. I do mostly city driving though, so that's probably why. I did make a trip up to Wash, DC and back (about 189 miles round trip), 90+ degrees, a/c on auto 74, a suitcase, the wife, kids, and the rack on, and my average was 50mpg, so I'm not at all seeing a disadvantage to a roof mounted rack Dave.
As much as I like my rack, I think the best thing about the Prius is the space with the back seat down. No need for the rack when I'm just running over to meet my buddies. FWIW, it was this Honda Insight story that scared me on hybrids and racks: "However, with 2 bikes on the back ( 2 Trek Liquid 25's) the drag interferes badly with fuel economy and top speed. I can average 50 mpg with one bike inside, going around 70mph--this drops to about 42 mpg when I'm averaging 80 mph. If I have the 2 bikes on the rack, 70 mph fuel economy drops to about 35 mpg, and 80 mph drops it to about 25 to 28 mpg. Similarly, it won't go much over 90 like this, whereas with one bike inside my Insight will do about 115 mph ( its a CVT , not the manual)." http://tinyurl.com/dusd8 It could be that the Insight is in a more critical aerodynamic state. Or maybe that was atypical ... I dokn't know.
I guess technically I was matching him. I had one mountain bike inside, and one on the rack .. 50 mpg. Were you carrying road bikes or mountain bikes Dave?
I just ordered a hitch mount from costal dave. I am going to put 2 mountian bikes on the back. Im taking a 5 hour trip one way 2 bikes 2 people. I will let you know what hapens.
