Rear Hitch/Bike Rack, Drop in MPG???

I can't give you precise numbers because that would really require a controlled experiment with two identical cars driven in a very similar manner over the same route. Just forget aerodynamics for a moment, and you will just have a reduction in mpg from the added weight of the hitch, bike rack and racks (about 125 lbs of additional payload for a single bike). I carry bikes on both the roof and on a hitch carrier and feel the mpg penalty is a less on the hitch and the roof but can't give you a good estimate. Just haven't driven enough with them to form an opinion yet.

My intuition tells me that stuff on the roof is the loser when you add areodynamics into the equation. Of course your speed will also play a role as city driving with stuff on top or on the back will have less of an effect than at highway speeds.

My only quantitative measurement was last summer where I was carrying two bikes and a Thule box on top and two bikes on the hitch mount at the same time. The car was loaded with two adults and two children and the trip was mostly at highway speeds (~55 to 65 mph). Our Prius averaged 38 mpg during that trip whereas I would usually expect around 48 mpg on a highway trip without all the added stuff.

 

<div class='quotetop'>QUOTE(eyeguy13 @ May 6 2006, 10:14 PM) [snapback]250996[/snapback]</div>

Yes. The best way to reduce the effect of lowered mileage due to a bike rack is to mount your rack on a Hummer. Then you've got NO worries.

And quite seriously - yes it'll lower your mileage. No, you shouldn't worry about it. Think of your INCREASED mileage when you're riding your bike instead of driving your car.