From one of our recent long travel excursions:
A word about fuel shopping. Being a couple of kinds of engineer, I analyze everything I can (boring, isn't he?). While on the road with DW at the wheel, I was looking ahead for fuel. (GasBuddy does this real well on a computer with internet, the phone app, not well.) We are still running SA2015 and it tells where 1/4 tank should be so I start looking there (and a state borders that are known to cause price changers). This is where the computer version shines, click the box in the corner to go to full screen and with SA to feed you Zipcodes it is real easy.
In one place I found fuel for 0.10$ less, but it was 10 miles off the route. That started a stream of thoughts that I shall relate here.
If you think about it an undue length, that 10 miles is more than a gallon for man of us. You will have to replace that gallon. Then, when you get back to you planned route, you have also burned a gallon more and that just decreased the distance to the next fuel stop by that gallon.
Short version: If you have to go out of sight of the route to save money on fuel, it is probably not worth doing.
Now, Costco, Sam's Club and Kroger all deserve their own analysis, because they can work. Even still that 0.30$ a gallon can be a looser if it is too far off your planned track.
Amazing what you can do with a spreadsheet while the DW is driving.
Matt