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Unread 09-26-2012, 08:57 AM   #1
Elihawk
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gas mileage question

2008 Jeep Compass, 5 speed, 2.4 liter engine. I consistently get 20-25 mpg in town (20 in cold weather) and 25-28 with mostly highway miles. Being I live in Iowa and farm country, I always buy Ethanol blend gas...

The other day, I stopped at a station where the Ethanol blend gas pump was out of order...so I filled up with premium gas. I noted the mileage and was suprised that on a highway trip where I usually get 27 mpg in the fall (speed limit is 55), with premium gas I got just under 22 mpg. I assumed that I would be improved over ethanol blend gas. Second time I have noted a drop in mpg with the highest octane gas available. What am I missing?

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Unread 09-26-2012, 09:40 AM   #2
FujativeOCR
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Tire pressure? Temps coming down in fall your tires might have deflated.

Also, Premium gas is for high compression engines. Ours are supposed to use 87, which is actually more combustible than high octane. Maybe we get more power iff regular because of this.
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Unread 09-26-2012, 09:53 AM   #3
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To expand on that, using "premium" gas, causes the engine to adjust it's own timing to compensate for the delayed combustion. Thus the engine produces less power, and probably requires more gas to compensate. Stick with 87.
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Unread 09-26-2012, 10:02 AM   #4
Elihawk
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Thanks for the explaination...as I said, I usually buy ethanol blend (89) and get really good gas mileage, so I will stick with that!
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Unread 09-26-2012, 11:34 AM   #5
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Just so you know...the Regular, Plus, and Premium designations are a sham to imply the higher octane is better. It's not better it's just designed for different engines. The octane rating is a percentage number of how much octane vs. heptane is in it. It's just a different blend with different ignition properties. But it' isnt technically better. It's just that usually the cars that need it are better cars. Cars with turbos and superchargers etc. if you put 87 in a HP car it will pre ignite and knock the engine. Conversely, putting 93 in our engine only makes it retard the timing and lower performance.



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Unread 09-26-2012, 04:01 PM   #6
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Premium (91 or 92 or 93 octane) gas is not just for higher compression engines. It can also be used for more power and better mileage if your car's computer is properly tuned for it using a programmer, without really raising the compression ratio. Unfortunately, there aren't really any programmers for our little 4 cylinder engines.
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Unread 09-26-2012, 04:06 PM   #7
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Ok....exceptions to every rule. But most people aren't engine tuners.
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Unread 09-26-2012, 05:29 PM   #8
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ethanol fuel mileage

Higher octane fuels will always have a worse fuel economy in ANY engine. The ethanol is used to boost the octane rating of gasoline to whatever it is wanted to be. IE 87,93. Pure ethanol has an octane rating of 116. This is blended to gasoline up to a maximum of 10% to federal limits. 87 octane gasoline has almost no ethanol in it. 93 octane will have closer to the 10% limit. Ethanol yields only half the fuel mileage of gasoline because ethanol needs half the amount of air gasoline needs. The more ethanol mixed in gasoline the worse fuel mileage. The amount of air, (ie oxygen) inside and engine cylinder is always the same. Unless the engine has forced induction. Twice as much ethanol is needed to get the proper air/fuel mix inside the engine for combustion as compared to gasoline. This is a big part as to why racing engines make so much power burning alcohol. Computers will sense the ethanol content of gasoline and compensate with more fuel to get the air/fuel mix right for combustion and emissions.
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Unread 09-26-2012, 07:56 PM   #9
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^that is incorrect. Where do you suppose ethanol free gas comes from? And how do you think it exists in all octane forms? 87, 89, 91, 93?
Gas is a mixture of Octane and Heptane. Ethanol is added (more often than not, but not exclusively) for emissions/pollution reasons, and yes the federal limit is 10%, except for E85. Gas with ethanol in it hurts gas mileage. But Ethanol has nothing to do with octane ratings.
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Unread 09-26-2012, 09:13 PM   #10
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Quote:
Originally Posted by cuemark8 View Post
Higher octane fuels will always have a worse fuel economy in ANY engine. The ethanol is used to boost the octane rating of gasoline to whatever it is wanted to be. IE 87,93. Pure ethanol has an octane rating of 116.
There are many different places that claim different octane ratings for ethanol. I've seen 99, 113, and 116 stated. The most common rating I see is 113, but they do not state if that is a RON, MON, or an AKI measurement.

In some states, they do use ethanol to boost octane ratings slightly. However, none of it is standard across the country (varies state to state), and the ethanol only impacts the octane a little bit due to the 10% or less guideline. What Fujative says has a lot of truth to it.

The only gas in my state of South Dakota, for example, that has any ethanol is typically the 89 octane, which is 10%. I have read, although I could not find any concrete evidence, that this is 85-87 octane gas that has 10% ethanol mixed to "boost" the rating to 89.

Also, South Dakota state law requires it be marked as containing ethanol if it does. The 91/92/93 octane here does not contain any ethanol unless it is marked as such. Many states have similar laws. Minnesota, on the opposite end of the spectrum, has a state law requiring all gas to contain 10% ethanol, or so I am told by relatives that live there. And this does not have to be marked on the pumps at all. They were even considering a law at one point to require 20% ethanol in gas sold.
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Unread 09-26-2012, 09:27 PM   #11
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California too. All gas is 10% ethanol. There's 5 stations in the whole state that have ethanol free gas. I don't know how they get away with that at those stations.
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Unread 09-27-2012, 06:42 AM   #12
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Find a station in your area. I have one in my town that has racing gas. http://pure-gas.org/
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