Best brand of gasoline?

This is an old thread, but I'll use it rather than start a new thread. I'm in Utah where the gas is 85 87 or 91 octane. I fueled up with 88 octane to get ethanol free to try it. On the freeway at 80 mph I achieved almost 54 mpg. The best I did before that was around 45mpg. That is a 17% increase which is what prompted me to find this chat to see what others have said about it. (2026 LE AWD) According to dollars, I was only getting a 13% difference in gas price so it doesn't quite make it cost effective to put the 88 octane in. UNLESS I am going on long trips between pumps. And yes, that is a thing here in the west, especially when going through Nevada. You can stop at the little gas spots, but expect to pay much more for "convenience" gas.
On a side note: last year I drove a lonely highway through Nevada at 100 mph in my Infiniti and went a full 20 minutes before another car went by in the opposite direction. Don't stop though, the government did hundreds of nuclear tests in Nevada.

 

That's a big change that you are attributing to the lack of 10% ethanol.
Do you happen to know the wind speed and direction that particular day? A 20 mph direct tailwind could be the cause....

I have a different 88 Octane story: We were coming home from 3 days camping in the Ozarks in our new to us RV based on a Transit van.
I bought 15% ethanol, 88 Oct gas for $2.52/gal, which was cheaper than the normal 87 Oct w/ 10%.
It had a placard claiming 'It's fine for 2001 and later'....
30 miles into that tank the engine started bucking and popping and loosing power. It was flashing the CEL. I pulled over. It stayed running.
The CEL went out. We continued and it didn't act up again. I haven't checked to see if it stored fault codes.
I'm guessing a small gulp of water came along with this gas.
Next town I filled it up with No Alcohol 90 Oct for max dollars.... It only took 4 gallons, but I personally felt better.

 

I did have the wind to my back on that trip, but even with wind before I couldn't achieve gas mileage into the 50's at that speed. You are correct though. Two things that will kill mileage in a Prius are wind and speed. Same for every vehicle, I guess, unless they have a lot of horsepower. They will still lose some gas mileage, but not to the percentage extent of a Prius. Hills also, but you can regain mileage on the downside.
And yes, it sounds like some water in the gas for your issue. Did you put Heat in the tank, or just refill? I don't know if condensation in the tank will be an issue or not this winter. I don't know much about the tank in Prius models compared to the older vehicles. I usually keep my tanks above 1/2 full at all times in the colder months.
Prior to this chat, I didn't know ethanol free stores longer. I'll be using that in my 4-wheeler now because it is only used as a snow plow.