From the online Navigation Owners Manual.
4.1.2 Route calculation and recalculation
The navigation system calculates the route based on your preferences:
Route planning methods:
FAST : Gives a quick route if you can travel at or near the speed limit
on all roads.
SHORT : Gives a route that has the smallest total distance of all possible
routes.
ECONOMICAL : This method combines the merits of the Fast and Short
routes: The navigation system makes a calculation for the Fast route,
but it also includes other roads in the calculation in consideration of fuel
economy.
EASY : Results in a route with fewer turns and no difficult maneuvers.
With this option, you can make the navigation system take, for example,
the highway/freeway instead of a series of smaller roads or streets.
I don't know if there is a default. It seems to stay on whatever method was chosen last, until changed.