I have only ever used 87 or 89 in my '07 Prius and do the same in my Prius c. Mostly, I use the 87 octane with less than 10% ethanol.
Pennsylvania has only 24 stations selling 100% pure(ethanol-free) gasoline. Camp Hill does not have any 100% pure gasoline. Go to pure-gas.org to find addresses, details, & a great zoomable map to the minority of 5400 stations (1 in every 32) that sell 100% pure gasoline in the U.S. & Canada. If you get to either Dover or York Springs, extensively test 100% pure gasoline. Switching to 100% pure gasoline, our 3 cars' mpg increased 8.3%, 7-8%, & 5%. All engines run smoother, quieter & with more power, such that, less downshifting is needed to ascend hills. pure-gas.org has lately been showing an increase in stations selling 100% pure gasoline. Part of that is that ethanol subsidies ended at the end of 2011. However, pure-gas.org is getting a good reputation for showing where stations are selling 100% pure gasoline. One recent individual for a fuel supplier, in an effort to increase their business, has been posting their stations as having 100% pure gasoline, when they do NOT have 100% pure gasoline. pure-gas.org is correcting the situation, but often bad people can screw up a good system. Premium gas is NOT 100% pure gasoline.
Unfortunately, it is not worth the inconvenience of driving to Dover or York Springs to get 100% pure gas, not to mention the difference gained in the gas mileage is just not enough to justify the cost of my time to drive to fill up. I appreciate the information, though.
#weoh--- Toyota does more than not recommending anything more than 10%. From the owners manual Page 107: Use only gasoline containing a maximum of 10% ethanol. DO NOT use any flex-fuel or gasoline that could contain more than 10% ethanol, including from any pump labeled E15, E30, E50, E85 (which are only some examples of fuel containing more than 10% ethanol). Also, in the back page 552: Unleaded gasoline, Octane Rating 87 (Research Octane Number 91) or higher It does state that the advertised MPG's are based on using 91 octane fuel.
My corner BP is changing to a Valero, which is new brand in our area, so it looks like I will be switching to this soon.