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There seems to be quite an amount of confusion over whether or not it's necessary to follow the manuals and fill up with 87 gasolene even at an elevation where 86 and 85 petrol is being sold as Regular.

When the discussion comes to whether there's any benefit to use Midgrade/Plus 89 or Premium/Supreme 91 in place of Regular 87, when the car manufacturer specifies 87, the consensus is that going with the higher graded fuel is simply a waste of money in most circumstances.

However, when it comes to 85 and 86, the picture is not as clear cut. Some claim that there was a major benefit of 85/86 many decades ago, whereas modern cars don't get any actual benefit. Others claim that it's actually detrimental for modern cars to not have the spec'ed 87 — the computer could still adjust, but you're going to lose fuel economy and power in the end.

What's going on?

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As a starting point, this is what I found in regards to 85 vs. 87 in Colorado:

http://www.denverpost.com/2006/10/16/some-knock-states-lower-octane-levels/

Research several years ago from the American Petroleum Institute showed that lower air pressure at higher altitudes allows vehicles to perform as well with 85 octane as they would with 87 at lower altitudes.

But a 2001 study by the Colorado Legislative Council, the state legislature’s research arm, concluded that the altitude difference may apply only to older cars.

“Research findings indicate that newer vehicles manufactured in and after 1984 are equipped with sophisticated electronic control systems that minimize this altitude effect and may perform better using higher-octane gasolines,” the report said.

I couldn't find the original report, but, frankly, after going back to the 87 and 89 debate, isn't the whole and only reason to use 89 is that it provides "more power"?! Is such loss of power with the use of 85 the only consequence that they've found?


However, this is what https://www.fueleconomy.gov/feg/octane.shtml#85 says, although no specific sources are cited for the very claim w/r/t/ the effects of the elevation:

The sale of 85 octane fuel was originally allowed in high-elevation regions—where the barometric pressure is lower—because it was cheaper and because most carbureted engines tolerated it fairly well. This is not true for modern gasoline engines. So, unless you have an older vehicle with a carbureted engine, you should use the manufacturer-recommended fuel for your vehicle, even where 85 octane fuel is available.

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