I have a 2013 Lexus ES300h ... and very very satisfied ... reliability is the best and after more that 70,000 km, I have not been in a garage for any repairs ... only cost is oil and filters changes.
My point is that my car MPG is very different depending which brand I use ...
I tried most gas brands in my city (Ottawa-Ontario-Canada) and I found that there's is quite a big difference between gas companies.
In about the same summer weather conditions, I can average 45-54 MPG on the highway, depending on the average speed, with good gas from Mac Ewen or Irving brands and an average of 37-42 with some independent stations.
I was surprised to find out the big difference and I always suggest to my friend to also try different gas stations and calculate their consumption as there can be 15-20% difference from brand to brand.