I don't have the NAV so when I need directions, I look at Google Maps.
Not sure if you are aware, but Google Maps recently (okay, it may have been a few months ago) added a very neat feature.
As you may know, Google Maps allows you to get directions between any two points.
But what if you don't like the directions?
What if you want to make a detour?
You can now drag the route line and drop it anywhere you want and it will instantly recalculate the mileage, drive time (which is always incorrect and, most importantly, turn by turn instructions).
Here is a
link to take you from Beverly Hills to New York.
Hover anywhere on the blue line and you will see a white circle.
Find a point along the line to 'pull' and drag the line and drop it on a city. Make sure you see the tool tip pop up that says "Drag to change route" or you will be dragging the map instead of the line.
It is a very cool feature!
(not sure if other mapping services have this, or not)