Found a cool web site that will plan your route based on lowest cost fuel stops along your way.
http://www.gasbuddy.com/Trip_Calculator.aspx
It says it will save me $140 in fuel going from PA - CA - PA... thats about 3 tank fulls.
It doesn't route you by shortest distance, rather by lowest cost fuel stop to lowest cost fuel stop.
So you may run up a few more miles, but it will cost less overall by avoiding the high priced gas along the way.