If you're one of the millions of Americans preparing for your summer vacation, you might also be readying the family's bicycles for some exercise while you're away.

Chances are, this involves some kind of roof-mounted bicycle rack for your car. And this can have a dramatic effect on your gas mileage.

Consumer Reports has calculated just how much extra gas the aerodynamic hindrance of a bike rack can cause, recording figures with a 2013 Honda Accord LX.

The 2.4-liter four-cylinder vehicle, capable of 29 mpg combined with an auto transmission and 28 mpg combined with the manual, achieves 30 mpg in CR's own testing.

Better still, at a steady 65 mph it recorded a test baseline of 42 mpg--against which the effects of a bike rack can be measured.

The differences are surprising. With a rack alone, that 42 mpg Accord drops a full 5 mpg, down to 37 mpg at 65 mph. With a wind deflector--for the bikes, rather than to reduce the drag of the rack, that drops further to 35 mpg.

Worst of all, a rack with two bikes and a deflector returns just 27 mpg at 65 mph, a 15 mpg or 35 percent drop compared to the bike-free vehicle. Throw in the weight of a family and all their luggage, and you'll see that figure tumble further.

These figures will vary depending on your cruising speed. A trip across town, with lower speeds and a shorter journey time, may not impact your finances much, but a 15 mpg drop (or more) over several hundred miles will start to cost you. You can mitigate this to some extent--dropping your speed, or driving more gently--but the bottom line is that anything external to the vehicle will really ruin those finely-honed aerodynamics.

Obviously, there's little alternative if you want to take your bikes away on vacation--short of occupying space in the car itself, they'll either be going on the roof or attached to the back of the car. And at least using the bikes once you arrive is a green (and healthy) means of getting around.

But once that vacation is over, remember to remove your bike rack--otherwise you'll be paying for that vacation for the rest of the year.


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