Hey Scotty, I recently moved from the Midwest to the Rocky Mountains. Out here, regular fuel is 85 octane. They claim you can use it instead of the 87 recommended by the manufacturer because it burns differently at higher altitude. Is this true? I've stuck with 87 octane, which is sold as mid-grade here, because that's what Toyota recommends.
you didn't mention what you're driving , but modern cars will adjust to the fuel and air pressure. They will just make a little bit less power that's all. If you don't notice any major problems on 85 octane then don't worry about it.
@mountainmanjoe
Sorry, 15 Highlander and then we also have two Corollas, 09 and 11. Not worried about the Corollas, those things will run on anything. The Highlander is my baby though, might stick with Toyota recommendation on that one!
That may be a true claim as to why they are selling 85. The higher the altitude, the air is less dense and therefore less oxygen in a given volume. The octane rating (gasoline's resistance to auto-ignition in higher pressures) coincides with the amount of oxygen your engine consumes. Turbocharged and supercharged engines use a minimum of 91/93 because more air is forced into the cylinder (more pressure). The more air you get in the cylinder, the higher the compression. You need a higher octane gas for higher compression to prevent knocking (prevent the gas from pre-igniting due to the high compression). And conversely, you use a lower octane gas for lower compressions. But here is the thing, you can use higher octane gas for lower compression (may or may not feel a difference), but you CANNOT use low octane for higher compression.
In your situation, you're getting a lower compression than normal because of the altitude you are at (less air pressure), so you can use a lower octane rating like 85 without the risk of engine knock. You can try out 85 and see if there is a difference. If there isn't a difference then just use 85 since it's the low-grade there and cheaper.
Fun fact: your engine actually produces less horsepower at higher altitude because of this. Engines produce the most power at sea level. This was highly researched during WWI and WWII because of the introduction of aircrafts.
