Octane rating?

There is no advantage to using super in a vehicle that is rated for 87 octane. Super actually is slightly less combustible (anti knocking agents) and is suited for higher compression engines. You will not gain ANY MPG using super. About the only thing you will accomplish is increasing your fuel bill.

 

No Prius engine was designed for a higher octane rating than regular fuel, so, in theory, there will be no performance or MPG improvement with higher octanes.

In practice, Bob Wilson found a minor power gain in his First Generation Prius when doing hill climbs, but not an MPG improvement. I am not aware he has replicated that in a Gen 3 like you have. As an engine ages, carbon build up may introduce ping or knock, and if you notice it, higher octane fuel will be cheaper than an engine rebuild.

 

The gen 3 Prius keeps cylinder pressure pretty low to keep the combustion temperature down and reduce Oxides of nitrogen emissions. Even so, My gen 3 gains a little fuel economy(2 mpg) by going from a known quality 87 octane pure gasoline, to a 90 octane E10 blend using that exact same quality gasoline.(both the E0 and E10 from the same blender pump.) I dont know if the slight mpg bump is the result of myPrius being able to tune ignition timing a bit higher to get a little more efficiency out of the fuel, or if it just is more efficient at converting that particular fuel mixture to cylinder pressure, which is what determines power and fuel economy. I read the effective compression compression ratio of a gen 3 Prius is a bit below 10/1, so it should not benifit from an octane rating of more than 90.
In a Prius, I suspect the ability of a given fuel to combust efficiently is more important that high octane.
I have seen as much as 8 mpg difference between a really bad fuel and a good one that my Prius "likes, although they had the same octane rating, so try different blends from different stations until you determine which one your car "likes".

 

Generally regular gasoline is 87 "Road" Octane (R+M/2 also known as AKI anti-knock Index). Some states have different octane values eg; due to elevation. Your Prius is designed for 87 Octane, as stated in your operating manual. Higher octane is generally not supposed to be better but as noted above some owners may see an advantage in special cases. According to my Gen2 maintenance guide, too high octane (Premium) may cause hard start problems, but I have not seen a problem the few times I accidentally used it.

Ethanol reduces 1-2 MPG so if you can get ethanol-free in your state you may see improvement

 

"Ethanol reduces 1-2 MPG so if you can get ethanol-free in your state you may see improvement" Yes correct and it also states to avoid Ethanol if possible in the Gen 3 . Also the ratings are minimum, so you could get a higher rating than 87 at anytime. I know Texas has 4 different octane ratings at some stations, but they say just use the 87 and it will run fine.

 

Well my mistake I should have said E10 (10% ethanol) causes 1-2 MPG loss. Prius is designed for 0 to 10% ethnaol, and many states mandate E10 so you have no choice.

If you are a fan of ethanol, and you want to use E85, that is advanced topic (not recommended) but some here know how to use it. In that case you will lose more MPG like 30%.

 

I normally use 87, but I've also used 89 and I couldn't feel the difference in performance and didn't see better MPG. If you want better MPG, just let your Prius glide as much as possible whether you're driving on the highways or local roads. I have achieved 75mpg on trips around town but I could only get this when there were no cars behind me.

From what I read, higher octane gas is designed for engines that run hot and per gallon, high octane gas actually contains less energy than the lower octane gas. Since the Prius engine is design to run at a much cooler temperature, you shouldn't need anything over 87. I also read that the Prius computer limits the engine's RPM to 5000 max.

 

Gen 1 4500 RPM
Gen 2 5000 RPM
Gen 3 and v 5200 RPM
c or PHV no idea