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Mixing different octane.. bad?

My car requires 91 octane according to the manual and the gas cap. Costco I fill up at only has 87 and 93 octane so I always fill up in 1:2 ratio. ie. To fill up 18 gallons I would put 12 gal of 93 and 6 gal of 87 to get the exact average of 91 octane. Sounds tedious but I do save $3.50-$4 per fill compared to putting all 93oct. Does the gas in tank get properly mixed to come to the correct octane or am I doing anything bad to the car?

No harm at all. And as long as ‘required’, and not ‘recommended’ is the word used, you’re doing the right thing.


As Texas said. Does your manual say required as opposed to recommended? You don’t say what this vehicle is so we can’t really say but yes it will get mixed just fine.

2016 Acura MDX

What is the difference between required and recommended?


Congratulations on using the manual. Not everybody does that.

Sounds like a good plan. It should work to cover the advice found in your manual and save some money.

I grew up with the old Sunoco brand gas stations that let customers blend their octane at the pump. Underground were 2 tanks (high octane & low octane) and the customer would move a dial and select octane ratings from a range of choices. Of course the gasoline was priced accordingly, higher octanes costing more than lower.


Required: If you don’t, you can expect engine damage.

Recommended: if you don’t, you may lose power and/or MPG, but no engine damage.

Somewhere in your manual it might state 91 octane recommended ( That means you will possibly get the most power and mileage you can get ) it might also might have a number such as 87 octane at least.
Seriously you need to call the dealer service department as they are the ones who will handle warranty claims.

Good way to get your 91 octane. As @common_sense_answer says Sunoco used to do this with their blend selector.

I have no idea if this matters or not, but I’d always put the 93-octane in first, as that’s what’ll be picked up at first if the fuels don’t mix until you drive around for a bit.


You can use 87 octane in this vehicle, unless you really need 300 horsepower;

Fuel Information
Use of unleaded premium gasoline of 91 octane or higher is recommended.
• Acura recommends TOP TIER Detergent Gasoline where available.
• Do NOT use gasoline containing more than 15% ethanol.
• Do NOT use gasoline containing methanol.
• Do NOT use gasoline containing MMT.

“• Acura recommends TOP TIER Detergent Gasoline where available.”

@Nevada_545 posted that information about the Top Tier gasoline recommendation.

Most people are using top tier brands.

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Even if you car was required to have 91, the 93 would be fine. So unless you really need to save the $4 a tankful, the only thing you’re doing is wasting time. Both yours and the people waiting. In line behind you.


Most gas stations will sell 3 (or more) levels of gas - typically stations near me sell (87, 89 and 91) Octane. However the station only has two tanks (87 octane tank and a 91 octane tank). To achieve 89 Octane the 87 and 91 are blended at the pump. So you mixing is exactly what the gas stations do anyways…Not harmful at all.

You can just fill it up completely with 93. It’ll only be a few bucks more and the engine will probably run even better. There’s no need to try to mix down to exactly 91 - the recommendation is 91 or above. You won’t hurt anything doing this, but you’re inconveniencing yourself at the pump every time you fill up and I imagine you will eventually get tired of it. And the people behind you in those interminable Costco gas lines will get annoyed if you take extra time doing that.

Plus, you may find that the manual is written in a generic ,averaged, manner .
Yet we all know that octane numbers vary by altitude.
So . . depending on where you are ?
that 91 in the manual may very well be exactly the 93 at lower altitude.

There should be ZERO difference in how it runs or fuel economy

Since we don’t have Sunoco out here I was unaware of that. My first 2 service stations were formerly Hancocks and had the blend pumps. A few regular customers had a favorite blend. Everyone else just asked for regular or premium.

What leads you to that conclusion?

If your vehicle is designed to run on 91 octane…then using 93 octane won’t do a damn thing. Octane rating is measure of the fuel to resist knock…END OF STORY.

Not quite. Knock tendency varies with intake air temp.
Modern engines with knock sensors can run “close to the edge” with the specified octane.
In very hot weather the knock sensor will sometimes come into play.
Out in the net there was someone who did a study by recording data with an OBD scanner.
From my own experience I found when it’s above 95F my Matrix, which specs 87 octane runs a little peppier with 89 octane.
I haven’t recorded data but I suspect it’s because the spark doesn’t get retarded as much.
I only did this once as a test because the gas mileage did not improve and I didn’t deem the performance difference worth the higher fuel cost.