First, the wife read in a guide book that car navi cannot be trusted
there. The exact reasons were not described, but I would think barring
being in a deep valley, there would be no reason for it to be any
better or worse than anywhere else.

Second, what are driving times like? She found a web page that seemed
to use 50 kph as the basic speed for all the time calculations.

1st, that guide book is amazingly awful.  don't trust it for anything. 
there are plenty of cars here with car navi, and it works fine.

2nd, what the heck are you talking about.......time calculations???  sp0eed
limits here are the sama as the rest of Japan.　this is not Mars.