I was wondering if anyone can help with the question of how much extra drag a roof box will create, and if this will be increased substantially with 2 boxes? We have an OC bed so have a big bubble on the front of the van which is going to reduce aerodynamics greatly anyway. The van has a huge roof rack and easily enough space for 2 boxes either side by side or one in front of the other. Is it worth using these to save space inside? How much will the mpg reduce with 1 or 2 boxes mounted in the different ways as above? Thank you.