Best way to Carry Boats for MPG

[ohioboater]

I used to think that having a bigger engine meant less relative penalty for carrying stuff on the roof, since the mileage would only drop 1 or 2 mpg versus 5 to 8 with smaller cars. Then I did the math.

F-150: 16mpg, 13 mpg with boats. 19% decrease
Mazda 5: 26mpg, 21 mpg with boats. 19% decrease
Subaru: 25 mpg, 20 mpg with boats. 20% decrease

In general, it seems that a roof rack with gear on it means a 20% increase in fuel consumption regardless of the car. All of the numbers I used above were comparing a bare rack to rack with gear. With no rack, the change is 25% across the board.