If you make the engine produce more power out of the same volume of fuel, then the power increases along with the mileage…to a point though…

When the restrictions to airflow and internal power losses due to friction and harmonics are removed and the rpms are decreased (Engine produces more power=less rpms needed for same amount of work) then the efficiency of the engine increases and power is released to move the Vanagon not the inefficiencies to produce better economies.

And as far as mileage, I am reporting on my area, where area where I drive my Vanagon in the NC piedmont region.
I fill up, drive 80+ miles, then fill up at same location, same pump.  The only variable is the temp outside during fill up (I can not control that)

Plus I got the five speed tranny as part of my upgrades...add $2K that to the conversion price.

Robert