Which grade of gas do you use?

I used to know the exact 'technogeek' answer, but the value listed in the manual is based on a different measurement than the goofy math formula that you'll find on the gas pumps here in the US.
Since this is PC, somebody will have the exact method of octane measurement committed to memory and you'll see it in another post below this one very shortly. They might even beat me to the answer.
Some people live for that kind of question.
The only reason more people don't ask this particular question is that you're one of those 1/1,000,000 people who actually read the owner's manual......even in the Wonderful World of Prius.

Bottom Line: If you're location is in the United States, and you're getting gas at less than about 5,000'? Use 87. I prefer a "Top Tier" gas station, but I've known people who put lunar mileage on vehicles using the cheapest gas that they can find. They sell Priuses all over the world. It's not going to make any measureable difference.
I've read that if you're in the mountains (some states sell 85) you can get by with an even lower octane than 87, but I like things nice and simple.
Use 87.
Don't worry about E-10 versus E-0 (Ethanol) and don't be one of those idiots who try to get by with using a quarter tank of E88 to try and save $3.44 on a tank of gas.

Good Luck!
And.....Welcome to PC!

 

Idiot huh? Yea I am one of those that have blended but 87 or 89 will be just fine.

Mike

ASUS Transformer Pad TF300T ? 2

 

The 88 figure in the manual is an error. Toyota stated that 87 is correct for the NA version of the Prius.

Tom

 

Assuming OP location is USA sea level, 87 Octane (R+M/2) Regular is the recommended fuel.
R=so-called Research Octane No. (RON) M= so-called Motor Octane No. (MON)
These are the actual laboratory engine test methods, whereas the Research Octane test simulates low load highway style (typcially R= approx 91) and the the Motor Octane test simulates driving with a load on the engine (typcailly M= approx 83) so R+M/2 = 87 at the pump.
Kind 0f funny names for the tests "Research and Motor" we'd all understand it better if they just called it Low Load and High Load. The average 87 is Pump Octane sometimes called Road Octane or AKI antiknock index.
If you live in the highest mountain locations, you can get away with lower octane grade and it is sold there.