Any ideas on what kind of mileage hit a roof box does to an E70?

[Spanky Deluxe]

Reports of mileage hits for roof boxes seem to vary wildly and really depends on the car so I'm interested in anyone's real life observations for an E70 X5. I know it's a bit difficult to judge because if you've got a roof box then you'll likely have more people and a lot more luggage weight in the car that'll decrease mileage anyway but still, any experiences welcomed!

Does anyone have any experience in roof box mileage? Interested in particular in diesel experiences but petrol reports would be useful too - it's the same aerodynamical shape after all!

I'm looking at a Mont Blanc classic vista 540 litre roof box, seems competitively priced with loads of storage.

[PersonaNonGrata]

I have not yet tried our Thule Atlantis 1800 on my E53, though I have OEM bars for it, we have used it on our Audi C6 A6 Avant. We did not notice too much difference; maybe 1 or 2 MPG decrease. They are quite aerodynamic. I believe the thing to consider is how much overhang you have over the front windscreen. The more that hangs over the front, the more noise and the great negative effect on mileage. Of course, you have to make sure that the rear doesn't impede the upper hatch from opening so you want a box that fits well between that space.

[Spanky Deluxe]

Cool, thanks for the feedback. My hopes are that since 4x4s aren't exactly the most aerodynamically efficient shaped cars out there that the impact of having a roof box on top wouldn't be too bad. I would imagine that the impact would be far more serious on something like a low down M3.