$ vs. Octane vs. MPG discussions have many variables, aka, opinions; including, but not limited too, the laws physics, driving style, vehicle maintenance.
Years ago I had a motorcycle that very clearly ran better with high octane Chevron gas. That started me paying attention.
The further you drive in a stretch without stopping the better your gas mileage. The more constant your speed, the better your gas mileage—with caveats. Slow down on up-hill grades and gently return to your original speed. Yes the cruise control is used, but I watch the engine speed more and adjust the cruise control accordingly, not allowing it to bolt up hills. The numbers will average themselves, no need to run the tank dry—very bad. Record the gallons and miles every time you fill up. Divide the miles into the gallons. Take those MPG add them up and divide by the number entries on the trip. This equals you mean average. If you are a fanatic—like me—or just bored, you can record the time.
My '90 Legacy LS got 37.1 MPG at 62.4 MPH average speed; mostly flat terrain over 1229 miles; high octane fuel; stopping only to refuel. More recently, on a shorter trip, it got 35.2 MPG, high octane fuel. Long coasts to a stop; accelerating slowly.
The most expensive part of your drive is accelerating from 0 to 1 MPH. Once you are in motion you tend to stay in motion—it does not cost as much gas to go from 1 MPH to 2 MPH. Accelerate as slowly as traffic will allow. Coast to the limit line; do not rush it. Remember the 'Speed Limit' is a Limit not a Prescription. Coasting = Positive Infinity MPG vs. Stopped / Idling = Negative Infinity MPG.
Apologies for the Rant.