2015 Dodge Challenger SRT
Wow ... that's incredible. I wasn't aware of any V8s getting that kind of "combined" mileage. It's not unusual to see those numbers for mostly highway ... but not combined. My '15 SRT 392 has put up some big numbers on trips, but not in every day combined driving."..... Oh and Wizard, not a huge deal but in real world driving I usually averaged 23.5 to 24 in the Mustang using regular fuel and 18.5 to 19 in the scat alternating between 89 and 93.
And as others have said ... stick with just 93 octane in your 392ci/6.4L HEMI. It's what the ECM is expecting. And if you had a 345ci/5.7L HEMI, then 93 octane would be a waste without a custom tune as that ECM is expecting 89 octane.