Gas Octane & High Altitude Areas

I have a question regarding gas (no, not that kind of gas, but gasoline). I used to live in a lower altitiude area where the low grade gas was 87 octane, but I moved to a high altitiude area (4,000 feet) and the low grade gas here is only 85 octane. I have heard that with the higher altitude and thinner air that you can use a lower octane gas and the cars run just fine. I have done some research on the subject but haven’t found anything definitive.

My question is that both of my cars requite a minimum of 87 octane gas per the car makers. I know you’re wasting your money by using a higher octane than your car requires, so is 85 octane gas okay to use at higher elevations even though the owner’s manual calls out for 87 octane? I’ve been using mid-grade in the interium as saving $0.12 a gallon isn’t worth it if I damage my car’s engine. Can anyone clarify this one? Thank you.

You can use the 85 octane fuel because at higher elevations the compression pressure, cylinder pressure, is effectively reduced. A large part of the Rocky Mountain West runs on 85 octane fuel.

“I have done some research on the subject but haven’t found anything definitive.”

You don’t have to do any research. When I lived in the high desert of Arizona (5500 ft) all the gas pumps offered 85 octane. I used it with no problems at all. Don’t fill up with 85 octane though and go to sea level altitudes and expect the same performance.

When you pilot your own light plane you find the neccessity to adjust the air/fuel ratio with altitude because of the thinner/thicker air.
If you left it alone you end up running rich ( too much fuel for the air ) at altitude or too lean when you cam back down. To keep tabs on this phenomenon they have an exhaust manifold temperature gauge. A lean condition causes the temp to rise.

Old carbureted cars and trucks would clearly see the difference too. My 1980 Bronco would run hot just going from here , 6500 ft , to Albuquerque ,5000 ft. Going on a long trip ro Ohio would require me to get out the screwdriver and adjust the carb.

The new computerised cars do the adjusting for us and you’ve seen that the gas companies have done some as well.

If I had a turbo engine I’d stick to the recommended octane.