An interesting side effect (no pun intended) of larger rims and tires,
seldom discussed, is rotational inertia.  One of the Canadian autosport
type shows "Sports Car Revolution", did up a road racing Acura.  When they
fitting larger rims, and lower profile rubber, they stayed very close to
stock rolling diameter.  They found however, an 11 HP loss to the wheels
when dyno testing.  The reason?  The larger rims and brakes added more
mass, farther from the center of rotation, and thus required more power to
accelerate.  Granted, once accelerated, the main issue is driveline and
aerodynamic loss, however most of us don't drive very often in a steady -
state situation.

Being that HP is at a premium in most vanagons, it's got me thinking in
favour of just keeping LT (truck) tires on my 14" alloys.  Food for
thought?