We're in the thick of summer travel season and vacationers are packing up their cars for getaways. Bringing bikes on vacation is a great way to get exercise and save gas touring your destination. But, our tests show that taking bikes with you will exact a fuel economy toll.

To find out how much fuel economy is actually lost with a bike rack, we tested various setups on a 2013 Honda Accord. We ran our traditional highway fuel tests with no rack, an empty rack, empty rack with wind deflector, and a rack with two bikes and deflector. The results were dramatic.

To test fuel economy, we use a precise fuel-flow measuring device spliced into the fuel line, then run two separate circuits. One is on a public highway at a steady 65 mph. That circuit is run in both directions to counteract any wind effect. A second is a stop-and-go simulated city-driving test done at our track. CR's overall fuel-economy numbers are derived from those fuel consumption tests.

The Accord LX four-cylinder used for this testing is exceptionally fuel efficient, earning 30 mpg overall--good enough to rival many small cars.

As we cruised at 65 mph in the Accord with a pair of bicycles on a roof rack plus wind deflector, our highway fuel economy plummeted by more than a third, from a miserly 42 mpg to a miserable 27 mpg. The rack alone wasted 5 mpg. Keep in mind that a car filled with people and cargo will also have an effect on fuel economy, so pack as efficiently as you can.

Going across town, this may not be a concern. But if you're planning a multi-state family road trip this summer, a mounted bike would have a real impact on your fuel costs. Plus, it may create some annoying wind noise.

2013 Honda Accord (4-cyl.) MPG @ 65 mph No rack 42 mpg Empty rack 37 mpg Empty rack and wind deflector 35 mpg Rack with two bikes and deflector 27 mpg

There are plenty of other bike rack options besides a roof rack. See our tips for choosing the right one for you.

Also, check out our guide to summer road trips for advice on preparing for your next driving vacation, including what gear to bring, how to prep your car for the ride, and how to get the best gas mileage.