Y'all know I've  wondered what the impact on fuel efficiency would be if you used a roof rack for bikes or put a "rocket box" on top of a Prius. Unscientific it may be, but there's a recent post on Gristmill that sheds some light on this topic:

We took our Prius over the mountains a few weeks back. I was looking forward to testing it at the extreme end of its design envelope, with a bulky cargo carrier to boot. This gave me an opportunity to see how much highway mileage would be affected by aerodynamic drag…

We nailed 40 mpg on the nose for a 260-mile trip that was 95 percent highway driving…

My wife uses the Prius to do a 60-mile highway commute twice a week. She consistently gets in the mid-to-upper 50s on these trips. Based on those numbers, I would say that roughly 20-30 percent of its highway mileage results from its very low drag coefficient.

To be sure, 40 mpg is still an impressive number–better than any other car but the Honda Civic Hybrid.

My point today, however, isn't to sing the praises of hybrids in general, or the Prius in general (after all, I seem to be holding out for a new high performing diesel Honda).

Instead, the Gristmill post verifies something I've experienced putting bikes on top of our Audi: the impact on fuel efficiency of putting something on the roof of your vehicle can be significant. In fact, I've found it to be as much as 25%, depending upon how many bikes I put in the rack.

So, my point today is to wonder why anyone would leave a rocket box on their vehicle all the time? Why sacrifice 20-30% of your fuel efficiency on an ongoing basis for storage capacity you may only need occasionally?

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