For vehicles (such as stock Vanagons) where the engine power is
marginal, you may also find that you can take a grade at 75mph (due to
the better torque available at that rpm), but at 55mph, you can't make
the grade without shifting down to 3rd gear.

There are many trade offs, not just fuel economy, when you are
traveling at a slower speed.

Personally, if I'm exploratory mode, I'm off on side roads, where the
speed limits are slower anyway. Many old US routes are nearly
abandoned by the hoards that stick to Interstate highways. Traveling
on US 50 can take you to fascinating sight seeing areas that are hours
from the Interstates. I actually found crossing Eastern Colorado and
Western Kansas much more relaxing and entertaining when traveling on
US 50, instead of I-70.