Periodically one should change the air in the tires, kick the mice out of
the cabinets, disconnect and re-connect all important cables/wires, kinda
like flossing your vanagon.  Mike can tell you why it may solve a
mysterious, intermittent, vexing problem.  Burning goat guts and shaking
feathers and bones has also been claimed to help.  But not with such
difficult problems of fuel economy, a perhaps lean condition that could be
swapping the rear springs and correcting the lean condition.