91 octane isn't readily available here in my area, 87, 89 and 93. Even though my Challenger is an auto, it's had premium from day one. I don't want the ECM remotely thinking it has to retard timing because of octane deficiency. Also, I think it's better in the long run for the engine, I get 16 city and 24 highway, car has 1700 miles on it.