It's not that simple.
For example:
Why better economy if you have a longer journey? Apart from fuel, tires, brakes and other moving parts wear out more. And then what did you save? What about the other expenses? Time?
There can never be absolutely the same road conditions between two points even on same road ...

There are economy routes in iGO (also, actually calculation - green), and again it is an approximate algorithm. How accurate it is ... who really knows