There are two times you'll notice a difference with the larger tires.
One time is from a dead start, but as the Vanagon really isn't a speed
demon, that's not a big deal to me. The more important time is at
highway speeds in 4th gear. Simply put the larger tires will make it
more difficult to maintain speed on up hills (flat terrain isn't a
problem). However, you'll have to have some really noticable grades (5%
or more). Since Interstate standards are supposed to be max of 6%,
you'll rarely encounter this, unless you happen to be in one of those
locales which got an exception (Western Maryland, on I-68 for
instance).