I was not told by my salesman about the premium gas preference, so I initially used just regular gasoline.
When I started to use Plus Octane (89) I noticed a 10% jump in MPG, using the internal computer system.
I have done this test several times on the same highway of just under 400 miles and my MPG change has always been the same; +10%.
Since the 89 Octane never costs more than 10%, I have kept using it with great results in power and mileage.