Yes it is all about wind resistance. I lose 3mpg with one bike on a roof rack on my SAAB 9-5 - it drops from 30 to 27 at 70mph, approx 10%. I would expect a similar loss on the TCH if you have them on the roof - possibly as much as 4-5 mpg with 2 bikes at 70mph.

If you mount them on a hitch-mounted rack it would be a lot less but still have some negative impact as you are still creating dirty air, even if it is behind the car. Might lose 2 mpg. I have not tried a rack on my Prius yet - I just stuff the bike inside - one reason I chose it over the TCH actually.