I use to run the cheapest gas I could find for years. My old 97 Volvo 960 121K miles, the fuel mileage over the years dropped from 29.25-29.75 mpg highway to the mid 26's. I had been blaming the drop on E10 for years.
With a friends recommendation, I threw in a bottle of Techron and switched exclusively to Shell V-Power for a few tanks. The cost was the same or within two or three cents per gallon of the cheap stations.
First tank, within 200 miles the engine smoothed out but I didn't see much increase in fuel mileage at first.
After about two and a half to three tanks of Shell V-Power my mileage jumped up.
Last road trip I took the first week December to Florida I got on average 29.2 mpg round trip.
BTW I don't drive over 65 mph due to wanting decent fuel mileage on a long trip.
I did the same procedure of a bottle of Techron and switching to Shell fuel in my 2010 Prius and
I've seen about 1.5-2.0 mpg increase in my highway mileage after a few tanks.
If it isn't Shell, nothing but a top tier fuel will ever be used in my cars from now on.
I'm completely sold on the benefits of Top Tier fuel.
The additive packages that these refiners add to their fuel do matter.