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Curious as to people's mpg results with roof rack.
I'm getting killed with my Yakima rocketbox.
35mpg vs. 45mpg without.
 

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I don't have a box type roofrack but I would expect at least 10% reduction. At least 5% just for support bars.

The increase of drag is the square of the increase in speed so the faster you go, the greater the effect.
 

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Here's another freebee, remove any extra weight in the car that you don't need and make sure the tires are inflated properly.

I remember the Mythbusters did a test where they compared AC to windows open at 55 mph and they confirmed that yes, using the AC is more fuel efficient even at just 55 mph because the engine drag is less than wind drag.
 

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Something on the roof really hurts mileage. Here's our trips to the Outer Banks for vacation:

With my 2003 Audi A4 Avant 3.0q:

2004: 1,572 miles; 25.80 mpg.

2005: 1,576 miles, 26.63 mpg.

2006: 1,408 miles; 23.31 mpg (bike rack and 1 bike on roof).

2007: 1,345 miles; 20.76 mpg (bike rack and 2 bikes on roof).

2008: 1,307 miles; 21.53 mpg (bike rack and 2 bikes on roof).

2009: 1,350 miles: 21.99 mpg (bike rack and 2 bilkes on roof).

With 2011 JSW TDI:

2010: 1,451; 47.76 mpg (both bikes fit in JSW due to significantlly larger interior storage).
 
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