Originally Posted by Bonnevillan
premium will not increase your mileage unless you are getting pinging currently
This is not entirely accurate. The knock sensors will not allow any pinging by retarding the spark. So you will probably never get pinging that you can hear. However, retarding the spark will have a major effect on fuel economy. Running regular in a supercharged 3800 will almost certainly require some knock retard therefore effecting fuel economy.
I have even noticed a difference in my normally aspirated SE by switching from 87 octane to 89 octane. I have measured a difference of around 1 mpg on long road trips.