Big non-aerodynamic vehicles with powerful engines barely see a dip in their already abysmal gas mileage if you add a kayak. Small cars, on the other hand, see a significant dip. For my Prius and Insight, the dip is about what you mention, gets worse if I try to go faster than 70mph, and no impact in the city, obviously.
That said, if I travel say a 1000 miles at 50mpg (40 gallons for the trip) vs 40mpg (50 gallons) at $4 per gallon, what's my saving? 10gallons=$40? It seems the rack will pay for itself over a few such trips...
But that math is flawed, because it includes the boats. I never drive far and fast with the rack on and nothing on it. At the same time, adding a boat or two on top I think negates the aerodynamic properties of the bars, so I would probably get the same mileage with loaded round bars or with foil-shaped bars...
So, the reaistic savings come if one can remove the rack when not in use - not paying for overly expensive rack and still getting your best mileage -;)
The real benefit is quietness, I think. Yakima sells aerodynamic plastic pieces that achieve similar effect that comes naturally to the Whispbar...
Cartop Kayak Carriers
Paddler's Truck Rack
Electric Kayak Motor
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