Claiming "my mpg is better now than then" is nearly impossible to nail down to a single factor such as fuel selection. Even getting your gas at a different station, or from a different pump at the same station can introduce variables.
Just because you set your cruise at "x" speed doesn't mean the drive was exactly the same. What's the elevation change between point a and b? Which was was the wind blowing? How much other traffic was on the road? What was the barometric pressure?
Given the right conditions, we could probably set up an experiment to show our cars getting better mileage than a hybrid.