The problem with the grades in Colorado is that they are so LONG. In
WV, the grades are shorter, but can be equally steep. In the eastern
section of the state, you have the problem that you keep encountering
the grades, about every 10 miles (in the Ridge and Valley section of
the Appalachian Mountains). In the western section of West Virginia,
the grades are more or less continuous. First up then down. In the
Eastern section if you are traveling in a northeast/southwest direction
you can travel long distances on relatively flat terrain, but if you
are traveling Northwest/Southeast, you hit those ridges every 10 miles.

Unfortunately, US interstate highways use a lesser standard than the
Autobahn in Germany. Grades are steeper (6% though waivers can make
that higher, versus 3%) and the roads are straighter (and more boring).

Vanagons hitting grades over 6% tend to not be able to maintain speed
in 4th gear. At least with stock North American spec engines. Even on a
6% grade, my syncros have trouble maintaining 70 with the stock engine.