My goodness you guys, nobody is thinking, and I can't sit quietly any
longer!

 The mass of the vehicle does not change the wind resistance, or the MPG, on
the highway. Stop and go or big hills is another story, but on flat ground
at steady speed the van could be 6 tons and you'd get the same MPG.

At steady speed you are not accellerating the mass, you are only pushing the
wind out of the way, so mass doesn't matter.

John
Aircooled.Net Inc.