85 in high altitudes?

I believe that it is generally accepted that lower octane ratings are ok at higher elevations. I know it was hard to find 87 octane gas when I drove across Nevada several years ago.

I don't think 91 octane will damage your engine, but the Toyota technical manual (used for training Toyota techs) warns that using higher octane fuel may cause starting issues with the Prius and also warns that the starting issues may not be easy to troubleshoot due to the multiple times that the engine restarts during normal operation.

 

Higher octane fuel than specified for specific car usually does no harm at all. But lower does.

 

Have a read of the following wikipedia link regarding octane ratings and refer to the "United States" section near the bottom;

[ame=http://en.wikipedia.org/wiki/Octane_rating]Octane rating - Wikipedia, the free encyclopedia[/ame]

This explains about the differences in altitude, but doesnt answer whether this is OK for the Prius. I guess it might help a little?

 

We split our time between CO and AZ. The lowest octane gas in CO (over 5,000 feet elev.) is 85 but in AZ (about 1,150' elev.) the losest octane gas is 87 at the same stations. The same difference in octane ratings holds true for the highest octane gas at those stations. That's because you can do just fine with slightly lower octane at higher elevations, i.e., the thinner air makes preignition less of a concern.

 

I could have sworn the owner's manual mentioned this.... if you're at a few thousand feet or more 86 or 85 are fine. It's the equivalent of 87 at sea level. 91 is a waste of money; if you need it to prevent pinging or knocking in a Prius it means the car needs a repair.