There are two (at least) factors that determine how much fuel you
will be using to go a given distance. One is the rolling resistance.
That is the amount of force that is required to push the van on a
level surface. This depends on things like bearings, gears, wheel
alignment, the type of tires, and tire pressure, multiplied by the mass
of the vehicle. The more it weighs, the more force you have to
overcome to flatten out the bottom of the tires and turn the bearings.
This is not affected much by the speed.
        The other main factor is the wind resistance. This is the
Coefficient of drag times the frontal area times the square of the
speed (or is it the third power? It's been a long time since I was in
school). This is (of course) very dependant on speed, and not on
weight. At 5 MPH, the power only has to overcome the rolling
resistance, but as the speed increases, the wind resistance
becomes a bigger factor. So increasing the weight will lower the
mileage a bit at all speeds, but increasing the speed will lower the
mileage more as it gets into the highway range. At around 50 to 60
MPH, the wind resistance becomes the biggest factor in how much
gas (or diesel) you burn per mile.
        All of that assumes that you are going at a steady speed on
a level road. Of course, accelerating up to that speed, or going up
hills will cause the mass to come back into play, so more weight
going up a hill will cause you to use proportionately more fuel. Since
Vanagons are not normally hybrids, you don't really get anything
back when you go back down the hills. All that said, I get around 18
to 20 MPG whether I am alone, or have 7 people and whether I am
driving on secondary roads or at 70 on the freeway. So, go figure.

Henry

87 syncro