Here's an old rule of thumb for octane, "The optimum fuel for any given engine is the lowest octane that won't detonate under the conditions you may encounter."

This rule may not work well for engines with knock sensors. They typically can retard ignition timing or reduce turbo boost to "tune out" detonation from lower octane fuel. This can result in a significant loss of power or mileage. This will be dependant on the individual engine & it's programming.

I recall a Car & Driver article from the dim past where they tested this. They had a turbo Saab with a knock sensor and tried several tanks of both regular & premium. The test drivers had a difficult time feeling a seat of the pants difference, but 1/4 mile times were almost a second slower with regular gas. Note that the power loss isn't from the lower octane, but from the changes in the engine programming to avoid detonation.

__________________
Norm in NJ
Next oil change at 230,000miles