Luggage rack's effect on mpg?

No first-hand experience, but the Prius MPG Simulator can predict the effect. At 55 MPH, you can expect about 57 MPG in 70F temps with no wind, on smooth drive pavement, level terrain, and at steady speed. Add a "small enclosed aero carrier" (as the simulator calls it generically), it drops to 50 MPG. A medium aero carrier gives you 44, and a large model drops it to 40.

 

Same here, no personal experience with a rack but I've heard that your highway mileage will take a good 10-20% 'hit' depending on the size of the rack. Around town where wind-resistance is less of a factor, the effect is less extreme, but you're probably not going to need the rack for around-town driving.

If it's just you and your husband, I'd think that the Prius ought to accommodate you well and be plenty spacious for a bunch of luggage. If you've got kids and are used to a mini-van, then you might be squeezed a bit, but the car hauls more stuff than you'd think from external appearances.

 

It would be nice if a hitch could be added and then one of those hitch mounted storage compartments could be used. This would cut down on wind resistance. Although I haven't checked I seem to recall a thread here about a hitch is either not recommended or prohibited by Toyota? Besides I suppose these carriers fit a 2" receiver and a 2" receiver on a Prius is a bit out of control.

 

Why do you care?

Occasional times justify the MPG hit. The other 99% of your driving, assuming you remove the rack then, will be far more efficient.

That's why I can enjoy the few indulgences I have with carrying kayaks on top. Those events are rare enough to easily consume the extra fuel for. And even then, the very worse case conditions result in 36 MPG. That's still way better than other vehicles.

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Point certainly taken. To further that point I could take my SUV at 11mpg and store everything inside on a long road trip... or I could take the Prius with a roof rack and still get almost 4 times the mileage even with the drag of the roof rack.

 

I lost about 5MPG with a rack on my 2001 Prius. I used this for snowboards, and it was not an 'aero box', just your standard snowboard rack with a wind deflector from Yakima.
With my new 2008 with the fold down seats I don't need the rack anymore so I just sold it on Craigslist.