For most vehicles/markets/climates the premium that's charged for 91 octane is not worth the return on fuel mileage. If 91 isn't required, I'd recommend everyone run a tank of both and determine which makes the most financial sense in their case.
To some degree. If an engine was designed from the outset to run on 87, 91 or 93 won’t suddenly get you better efficiency. But in the case of these newer DI engines, manufacturers are increasing the compression ratios (along with turbos) to add more power at lower displacements (That’s why a 2.7L V6 from 2024 will smoke a 5.8L V8 from 1995). So in those cases adding slightly higher octane fuel can increase efficiency. But you have to set down and do the math. If your efficiency increase, let’s say only 10%, but your gas now costs 30% more then there’s no reason to do it. You’d want to limit higher octane to just cases where you’d expect to see knock (towing up and down mountains, racing, etc). Where it gets interesting is when you tune these with help from the aftermarket. Those tunes can take into account higher octane fuel by playing with ignition timing, boost pressure, and fuel pressure to increase the envelop of the engine. As an example, the JB4 unit for the same 3.0L engine in the Braptor and RRaptor has multiple levels of tune depending on fuel. In a Braptor, that unit will add ~20WHP on 91, but if you’re willing to add E30 or E85, you can see gains in the ~50WHP range.
