This is a little more specific than the "wrong fuel" question: I'm looking for the long term impact of using a higher-octane fuel in an engine designed for lower octane fuel. Not quite diesel in a gas engine but tougher burning fuel... :)

This is the reverse of the question that is normally asked. Since premium burns under hotter / higher compression conditions than regular, could burning premium in my regular unleaded engine cause build up from incomplete combustion?

I was on Empty and found a gas station that only had premium left. Made me wonder if I was actually doing a disservice to my car!

  • 1
    Actually, it turns out that you've come to a place that cares very much about this sort of question!
    – Bob Cross
    Commented Feb 4, 2017 at 14:43
  • Thanks Bob Cross! The other question says that running higher octane in a lower octane engine would cause no problems, but what if I ran 93 in an 87 engine for 20k or 200k miles? Would I get deposits from incomplete combustion or would all the 93 burn when ignited?
    – Shrout1
    Commented Feb 4, 2017 at 16:11
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    No, you wouldn't get any deposits. Because once ignited, all the fuel will burn. You can always use higher octane fuel than what your car requires. Read about octane rating, it has nothing to do with how fuel burns: en.wikipedia.org/wiki/Octane_rating Commented Mar 11, 2017 at 19:09
  • Can you tell which car year/model is this also? It may make a small difference if it is an ancient car or a much newer car. Commented Mar 13, 2017 at 15:14
  • @EvrenYurtesen Hey! At the time this question was about a 2016 Dodge Charger. I also personally own a 2004 Pontiac and a 1999 Nissan :)
    – Shrout1
    Commented Mar 13, 2017 at 17:22

2 Answers 2


"Higher octane" only means more knock-resistant. It does not mean "harder-to-ignite" or "hotter burning."

A gasoline engine will not perform worse in any way on higher-octane gasoline. The higher-octane outputs of a gasoline refinery have only been reformed to be more knock-resistant. They still have to meet the same volatility, detergent, oxygenate, and other parameters as do "regular" gasoline grades for the same market. (And if you want to learn about all of the characteristics that are optimized and regulated, this technical review is fantastic.) In fact, you're likely to get a tiny (less than 1%) boost in mileage because the higher-octane C4-C12 isomers tend to be denser, so the higher-octane gasoline contains more energy per unit volume.

In theory it is possible for higher octane fuel to be less desirable: Ethanol is an octane booster and oxygenate that is significantly less dense than hydrocarbons. (Ethanol is also hygroscopic and can be corrosive to fuel system components not designed for it.) Given a choice between a fuel of adequate octane for one's engine, or the same fuel with its octane raised by blending with ethanol, one would be wise to buy the lower-ethanol blend. However I am not aware of any markets where premium gasoline contains higher blends of ethanol than the adjacent pump grades.


Yes it will burn as compleatly as lower octane, if the "octane" value is the only difference. But there can be different blends , sometimes the lower "octane" grades are worse though as they're there to be cheaper. The higher "octane" value only indicates the higher resistance to a damaging too fast explosive combustion .

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