The only accurate way to calculate mpg is to use one of the Trip history calculators in the EIC to measure miles between fills to full tank. Then divide those miles by the actual number of gallons used to fill the tank.I've had my Dart 1.4 turbo, DDCT for 4 months, have used all three grades, 87, 89, 93, No noticeable difference in performance. On one tank full of 93 I actually dropped 1 mpg from the tank of 87 that I had filled up with prior. 30 mpg to 29 mpg around town.
The most noticeable was when I started using the A/C, I dropped about 2 mpg.