Greetings Commander Cypher-Shryke
As far as I am aware the "Economical Route Planning Setting" Calculates a line of the shortest jumps to your destination. It uses less fuel however it often equates to many, many more jumps - especially over large distances - as such can take some time to get where you are going.
"Fastest Route Setting" will get you there far more quickly in the fewest jumps possible but this does use -a lot - more fuel.
When or if you pick a route that is outside your Fuel Range - The route planner will calcuate additional tanks and will choose an appropriate Fuel Star on the path to your destination that you can Scoop Fuel from.
In that case, its my advice never miss an opportunity to fuel up at any Fuel Star on route - or you maybe calling the Space Rats for help.
A thing to watch out for is should you be crossing over Spiral Arms of the MIlky Way as this is often a large expanse of Dead / Void Space ( some times over 100+ light years or more ). - in these situations its better to work you way close to the center of the 'Way and then work you way back up the Galaxy Arm to your destination.
I hope that helps,
Stay safe out there fellow Commander o7