[aerohead]

Ahmed Feysal did a Master's Thesis doing CFD on a Dodge pickup.
His research showed a drag increase when the airdam had between 6"and 3"ground clearance.At 3"clearance (for racing only!) he got a 1% drag reduction,which would be hard to see at the gas pump.
If the van behaves like the pickup,you may see an mpg penalty.
CAR and DRIVER modded a Ram Van in he 1970's.If I remember correctly,the only thing which helped them was a mirror delete.
Mirrors from a Tesla S would show an MPG increase.