87 vs. 91 Octane MPG and Performance

Generally speaking, burn the lowest possible octane fuel for which the car is designed.

The chemicals which increase octane tend to prevent the fuel from burning; in theory a higher octane fuel can actually reduce performance in an engine without sufficient compression (or the ability to adjust engine parameters for higher performance).

It's also cheaper, of course.

 

My v is perfectly happy with 87 octane, has 1000 miles on it now.

Peter+

 

87 octane is correct and you will see no increase in performance or mileage by going to 89 or 91. There are some tests by Bob Wilson on this site somewhere that bear that out.

If you are in the Rocky Mountain states above 3500 or 4000 ft you will find 85 octane, at those elevations it will do just as well as 87 due to the lower pressure in the combustion chambers.

 

My Prius and my Insight for that matter always run the 87 octane. Once I accidentally put 87 octane in my Lincoln Navigator... A vehicle designed for premium... Don't do that! It really didn't work out too well for me.

 

...keep in mind that there are different octane measurement scales.
In the USA we use average octane which = (R+M)/2
So USA Regular = 87 (R+M)/2 also approx. equal = 91 R(esearch) Octane (my Gen-II manual states to use Regular 87 and it mentions that this is also equal to 91 Research octane). So mid-grade is 89 Average octane. Premium would be 91+ Average octane. Generally stated no benefit for using higher octane than design. If your car is "knocking", and I have not heard engine knock in years for any newer car, then higher octane may be needed to stop that until you got the problem fixed. If I was climbing mountains, I might consider a dose of higher mid-grade octane for that day.

 

The good news/bad news is that the main reason you don't normally hear knocking or pinging on modern car engines is that they automatically retard the spark to eliminate it, normally before you can hear it.

 

I don't remember that but it could be, I'll try to find the thread when I get time. It would be good information to review. As I remember Bob put a lot of effort into it and he usually got things right.

 

Where is Bob when we need his input?

 

I tried a tank of high octane gas in my Gen III 2010 Prius and mileage decreased. Higher octane gas is more resistant to combustion, and so it does not fully burn in the Prius engine.

 

the 89 is marketing gimmick it does not exists. It is being mixed out of 87 and 93 at pump. Unfortunately many pump operators found that they can increase profit margins re-adjusting the ratio. Prius does not need it but if you truly need it, mix your own, 3.5gal of Premium for tank full (~10gal).

 

Here in the corn belt E-10 midgrade 89 is the cheapest. Interesting story, while we were waiting for our v to arrive my wife was driving my Chrysler 300C. She put gas in it thinking the 300C needed premium (91), it doesn’t. The MPG dropped to 15.5, after we ran the “good” gas HaHa. I put 89 back in it, mileage went right back to my normal 17 MPG.
Bottom line, don’t waste your money and MPGs on the 91+ octane gas. I have not read any material that says to run higher octane in a Prius.

 

You should burn 87 octane. 89 octane has less energy for the same volume.

The Prius engine doesn't have a knocking problem, so far as I know, and won't benefit from 89 octane.

Someone please correct me if I'm wrong.

 

Where does it say 91? Both references in the new car features manual are 87 or higher.

 

http://www.toyota.com/t3Portal/document/om/OM47674U/pdf/sec_06-01.pdf says 87 on page 3 and 12, however one could be confused by the RON of 91. (for countries that use RON)

Page 13 only says 87

http://www.toyota.com/t3Portal/document/om/OM47674U/pdf/sec_01-06.pdf also says 87 (RON of 91) on page 2

The US uses R+M/2 or AKI Octane, so uses 87
Octane rating - Wikipedia, the free encyclopedia

 

...well, just delete the second sentence about your energy content predictions, and you are good with me.