The only time I have noticed ANY difference between gas grades/brands/blends is when I filled up with "real gas" instead of the 10% ethanol junk. Within a few miles down the road, the MPG readings on my Scan Gauge were about 0.7 - 0.8 mpg higher than my typical MPG numbers. I didn't realize until afterward that I had filled up with gas from a state that didn't require ethanol blends. That works out to about a 10% boost in MPG.
~Rick