89 octane wrecked my fuel mileage

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A week ago when I went to fill up my car, as I took the nozzle out of the pump the hose smacked the 89 button which turned it on and I couldn't get the pump to switch to 87. I had enough fuel points that I was getting a free tank anyway so I went ahead and just got the 89. My fuel mileage for the following week was awful. My average was 11, and the highest MPG I got was 15 on a highway trip.

Now 2 days ago I filled up again, this time with 87, and almost immediately my city average went to ~16 and it got 24 on a highway trip. I'm not running a custom tune or anything crazy, so why did running 89 octane bring my MPG down so much?
 
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There is the possibility of phase separation. If that happened, you could have got a higher percentage of alcohol in your gas that caused lousy fuel economy.
The station you went to probably sells very little 89 and higher octane gasoline.
This will all be solved when we ditch the crap 87 and 89 octane fuels like the auto makers wanna do away with.
 
I normally run 87 octane in our cars . The Buick seems to get a few MPG better with premium , but does not seem to be consistent . When it is better , it does not equal the added cost , per gallon , of the premium .

Almost like it is not always the same grade of gas . This is buying from the same place .
 
I'm in Columbus Ohio and we barely got any snow. The wind wasn't all that bad either to be honest. We had freezing rain and my whole car got iced over but that's about it.
 
Originally Posted by cwilliamsws6
A week ago when I went to fill up my car, as I took the nozzle out of the pump the hose smacked the 89 button which turned it on and I couldn't get the pump to switch to 87. I had enough fuel points that I was getting a free tank anyway so I went ahead and just got the 89. My fuel mileage for the following week was awful. My average was 11, and the highest MPG I got was 15 on a highway trip.

Now 2 days ago I filled up again, this time with 87, and almost immediately my city average went to ~16 and it got 24 on a highway trip. I'm not running a custom tune or anything crazy, so why did running 89 octane bring my MPG down so much?



Something else caused the observed change, NOT the difference in octane rating.
 
Originally Posted by cwilliamsws6
I'm in Columbus Ohio and we barely got any snow. The wind wasn't all that bad either to be honest. We had freezing rain and my whole car got iced over but that's about it.

Is it possible that due to your car being "iced over" it spend more time idling to warm up before the drive? Combination of winter gas and longer idle warm ups could've done the MPG decrease.


Case on point:
- Flashback to my Uber days... My 2009 Scion xB consistently returned 25MPG average while Ubering in Charlotte area all day long at a rate of 1 fuel tank per day. So 5-6 tanks a week. Once it got colder - the longer idle times and "winter" (NC winter..hah) gas dropped my MPG down to 22-23MPG. Wasn't a one tank fluke either. Consistent MPG results, manually calculated (Miles driven/gallons filled=MPG) every day at 5am for weeks at a time.
 
I know in Iowa, the 89 octane is the same price as 87. The reason is that it contains a higher percentage of ethanol, which is a big deal in Iowa due to the corn. Anyhow, in a case like that, I could see the mileage being decreased compared to 87.
 
We have something that looks like data, but is missing some key numbers.

First, you give us MPG and units of time. I.E week = 11 MPG and then 2 days ago, some unknown distance highway driving yielded 24.

Are you talking about an MPG reading on a display or actual calculated MPG?

The true test is fill up at a specific pump and stop when it clicks off. Then return to the very same pump and follow the same procedure. Do some math.

That is about as close as you can get to quasi scientific.

Did the free gas fully fill your tank, or did you have a certain amount of credit in fuel points and once you exhausted it, the pump switched off? Is it possible you didn't fully fill up with 89?

I'm not sure you have enough data to warrant concern at this point. If this was a recurrent theme with known full tanks of 89, then yes, there might be an issue. But if it's a one-off, you may have had enough circumstances conspire, such as a not completely full tank, colder weather, ice / longer warm ups, or more heavy traffic and idling vs the tank of 87.

Or, you could have just got a bad tank of gas?

I wouldn't lose sleep over it unless it was a recurrent problem.

Originally Posted by cwilliamsws6
A week ago when I went to fill up my car, as I took the nozzle out of the pump the hose smacked the 89 button which turned it on and I couldn't get the pump to switch to 87. I had enough fuel points that I was getting a free tank anyway so I went ahead and just got the 89. My fuel mileage for the following week was awful. My average was 11, and the highest MPG I got was 15 on a highway trip.

Now 2 days ago I filled up again, this time with 87, and almost immediately my city average went to ~16 and it got 24 on a highway trip. I'm not running a custom tune or anything crazy, so why did running 89 octane bring my MPG down so much?
 
Because octane is not necessarily an indicator of an improvement in engine breathing and combustion, and since you drive an Engine Management Computer controlled car, he motor will notice and adjust parameters. Since it was designed for 87, which strongly implies the motor was designed for maximum fuel economy, any change probably means poorer mileage.

What makes you expect that higher octane than the OEM recommends means better mileage?
 
Originally Posted by Lenny5160
I know in Iowa, the 89 octane is the same price as 87. The reason is that it contains a higher percentage of ethanol, which is a big deal in Iowa due to the corn. Anyhow, in a case like that, I could see the mileage being decreased compared to 87.

No.

89 octane is made by blending regular 84 octane with 91 octane premium and usually 10% ethanol in the case of e10 89.
E0 87 octane is also made by blending regular and premium instead of regular and 10% ethanol.
 
True sometimes for 87 RUL E0, other times it's blended seperately and segregated from subgrade blends intended to be blended with ethanol. In the refinery I worked at in Ohio, we executed a project specifically for seperately blending and segregating 87 E0 as a way to debottleneck our naphtha reformer from having to do so many blocked high octane runs to make high octane platformate for premium that was then downgraded to 87 E0. Saved a bunch of money in energy and catalyst operating costs.

The refinery in Texas where I was the blendineer 87 E0 and 89 E0 were all blended individually, not by combining other blends.

At a storage terminal, blending 87 E0 from subgrade regular and premium intended to be blended with ethanol is the only way to achieve this.

All gasoline is a mixture of a bunch of molecules, sone give high octane but lower mileage due to lower density, some give higher octane and higher mileage due to higher density. At the pump what you have us the best mix the blender made for that batch with the ingredients available at that time with an eye to requirements for future blends on the schedule as well. There's no single set formula / recipe for any specific grade of gasoline at any specific refinery. Seasonal regulatory limits such as volatility change the recipes as well.

So one batch may have more low density ingredients that the pvetall mix meets the specifications, another batch for the same grade may have more higher density ingredients that the overall mix meets the soecifications. One should never expect on same basis (for example all E0) a tankful of regular to have lower mileage than mid-grade or premium or vice-versa.
 
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Originally Posted by Nyogtha
True sometimes for 87 RUL E0, other times it's blended seperately and segregated from subgrade blends intended to be blended with ethanol. In the refinery I worked at in Ohio, we executed a project specifically for seperately blending and segregating 87 E0 as a way to debottleneck our naphtha reformer from having to do so many blocked high octane runs to make high octane platformate for premium that was then downgraded to 87 E0. Saved a bunch of money in energy and catalyst operating costs.

The refinery in Texas where I was the blendineer 87 E0 and 89 E0 were all blended individually, not by combining other blends.

At a storage terminal, blending 87 E0 from subgrade regular and premium intended to be blended with ethanol is the only way to achieve this.

All gasoline is a mixture of a bunch of molecules, sone give high octane but lower mileage due to lower density, some give higher octane and higher mileage due to higher density. At the pump what you have us the best mix the blender made for that batch with the ingredients available at that time with an eye to requirements for future blends on the schedule as well. There's no single set formula / recipe for any specific grade of gasoline at any specific refinery. Seasonal regulatory limits such as volatility change the recipes as well.

So one batch may have more low density ingredients that the pvetall mix meets the specifications, another batch for the same grade may have more higher density ingredients that the overall mix meets the soecifications. One should never expect on same basis (for example all E0) a tankful of regular to have lower mileage than mid-grade or premium or vice-versa.



Thank you for posting a logically well written post.

This better then the "E-10 gives 10% less fuel economy" posts.
 
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