I just installed a Yakima car top rack and transported a 17' sea kayak to my house, a 50 miles trip one way. Without the rack and kayak, I was getting 'about' (dashboard reading) 51 mpg for the previous few days. With the rack only, I got about 50 mpg (60-65 miles per hour freeway). With the kayak added on the return trip, I got 40-43 mpg. I was driving slightly more cautiously on the return trip (55 miles per hour) because a 17' boat is a long load for the little Civic and the wind was catching it.

Edit: I consider the weight of the kayak to be negligible. Its 65 lbs is about the same as half a body stashed in the trunk . It's all about the wind resistance.

...because you'll see me passing everybody at 90 miles per hour going up mountains.