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Discussion Starter · #1 ·
Just took a 750 mile trip to Florida in my Jeep. I ran 87 Octane most of the way and got 18 MPG. At one point I decided to fill up with 89. Figured it would be the same or slightly better. My MPG dropped to 16.3 until I filled back up on 87. Bad gas or does higher Octane gas get worse gas mileage in our jeeps?
 

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Just took a 750 mile trip to Florida in my Jeep. I ran 87 Octane most of the way and got 18 MPG. At one point I decided to fill up with 89. Figured it would be the same or slightly better. My MPG dropped to 16.3 until I filled back up on 87. Bad gas or does higher Octane gas get worse gas mileage in our jeeps?
Usually higher octane = better MPG.

It could have been a variety of reasons:

1. Bad gas, as you said
2. Different driving conditions --i.e., more rain, a strong headwind, more curves, greater elevation, more congestion, etc.

Although going from regular to mid grade is normally not sufficiently different to notice; greatest difference is going from regular to premium.

Aldo
 

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I actually filled up with 93 just yesterday in my 3.8L. I normally use 87/89 but i like to use 93 octane from time to time since it's a cleaner burning fuel. I've done for years now. The higher octane since it is a "purer" fuel wll be burned faster than 87/89 so yes less mpg. But also with higher octane fuel the engine purrs much quieter and runs smoother and you'll notice a pick up in power. it's a very miniscule difference but it is noticeable...at least to me.
 

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Running anything other than 87 in your Jeep is a waste of money. Higher octane will always give you worse gas mileage in a low compression engine. The higher the octane the harder it is to ignite. This is done to keep high compression engines from pre detonating. That's why you got worse MPGs with the 89.
 

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Usually higher octane = better MPG.
Not in a low compression engine without timing adjustments to account for the delayed ignition.

i like to use 93 octane from time to time since it's a cleaner burning fuel. I've done for years now. The higher octane since it is a "purer" fuel wll be burned faster than 87/89
That is the exact opposite of what is happening... Higher octane rating increases the resistance to premature detonation under the conditions found in high compression / hot running / forced induction engines. If anything it is slower burning in your Jeep, and the same amount of it is getting burned either way.
 

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That is the exact opposite of what is happening... Higher octane rating increases the resistance to premature detonation under the conditions found in high compression / hot running / forced induction engines. If anything it is slower burning in your Jeep, and the same amount of it is getting burned either way.
Well $hit...hahaha good thing 93oct was only 2.21 at Sam's club yesterday. Oh well it'll burn away then back to normal

Thanks welder & C.L for the insight!
 

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Not in a low compression engine without timing adjustments to account for the delayed ignition.
Modern engines --Pentastar included-- self-adjust their timing based on the rate of fuel ignition.

Take your pick: MIT says higher octane fuel saves gas; CR says it doesn't.

http://newsoffice.mit.edu/2014/premium-gas-could-save-fuel-money-1028

http://www.csmonitor.com/Business/In-Gear/2012/0627/Myth-busters-Consumer-Reports-takes-on-alleged-gas-saving-tips

In the end, who do you believe unless you try it yourself, right?

Personally, I've found gains of 1-2 MPG using premium, but the savings are offset by the higher price of the fuel.

So, I guess, they are both correct.

Aldo
 

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Discussion Starter · #8 ·
Modern engines --Pentastar included-- self-adjust their timing based on the rate of fuel ignition.

Take your pick: MIT says higher octane fuel saves gas; CR says it doesn't.

Shifting up to higher octane | MIT News

Myth busters: Consumer Reports takes on alleged gas-saving tips - CSMonitor.com

In the end, who do you believe unless you try it yourself, right?

Personally, I've found gains of 1-2 MPG using premium, but the savings are offset by the higher price of the fuel.

So, I guess, they are both correct.

Aldo

Need to read a little closer. Because MIT flashes a big headline doesn't mean read no more. The big assumption is that manufactures redesign engines to run on higher octane in their simulation model. This is also a what if and not real research.

"The team factored in a policy-decision period of about three years to put in place a revised octane rating system, and an additional three to five years for manufacturers to redesign engines to meet the new standards."
 

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Running anything other than 87 in your Jeep is a waste of money. Higher octane will always give you worse gas mileage in a low compression engine. The higher the octane the harder it is to ignite. This is done to keep high compression engines from pre detonating. That's why you got worse MPGs with the 89.
Not in a low compression engine without timing adjustments to account for the delayed ignition.



That is the exact opposite of what is happening... Higher octane rating increases the resistance to premature detonation under the conditions found in high compression / hot running / forced induction engines. If anything it is slower burning in your Jeep, and the same amount of it is getting burned either way.
I will join the Choir...In my old TJ I used to burn up the Cats about every 10K-12K miles. Stopped running premium fuel and that issue went away. So now all my vehicle gets the 87 octane only....but all my vehicles do have the 3.6 Pentastar...sold the TJ a few months back.

There is a reason that the manual says to run the 87 octane. However the '13 Chrysler 300 we have is supposed to be a "flex-fuel" vehicle. I haven't tried that though............. OBTW YMMV GOOD LUCK!!
 

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Without using a tuner to make use of the higher octane, best case is it does nothing but waste money. Worst case is it wastes money and gets worse MPG and performance. The notion of higher octane equaling higher performance and MPG is a marketing tactic by the fuel companies hoping to get you to buy the outrageously more expensive fuel.
 

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Usually higher octane = better MPG.
My experience has been just the opposite if the vehicle is not tuned for the higher octane fuel. I have found that 93 octane DECREASED my fuel economy by 2-3 mpg in my old DD car that was designed to run on 87.

Higher octane is a waste of money if the manual calls for 87 but some people get that placebo effect, so maybe wasting money isn't as important to them.
 

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I wouldn't consider the Pentastar to be a low compression engine with it's 10.2-1 ratio. Most engines that have knock sensors will benefit from higher octane gas.
It may take a couple of tanks for the computer to slowly raise the timing from the lack of knock detected. I would think the programming would be based on averages over a period of time when determining timing to run. Kinda like the fuel trims do. GM's do this, not sure of Chrysler programming.
The ECM will try to run as much timing it can, up to the limit programmed. I doubt any engine running 87 will be at the upper limit. You'll need to log knock retard, with a scanner, to see how much timing is being pulled for each fuel. More timing means more power and better fuel economy.
 

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My experience has been just the opposite if the vehicle is not tuned for the higher octane fuel. I have found that 93 octane DECREASED my fuel economy by 2-3 mpg in my old DD car that was designed to run on 87.

Higher octane is a waste of money if the manual calls for 87 but some people get that placebo effect, so maybe wasting money isn't as important to them.
I am with you that with an engine that doesn't need it, higher octane may be a waste of money, because in the end the higher price of the fuel negates whatever performance results we get.
 

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I was always old school when it had come to fuel and I always ran premium. But, with a lot of counseling and continued education on this forum I broke the premium habit. I discovered that with newer modern engines, especially with a 3.8 or 3.6 in stock form, 87 octane is what should be used. There's no benefit to using anything else. I won't run no name gas. My 3.8 seems to like Shell and Sunoco.
 

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I wouldn't consider the Pentastar to be a low compression engine with it's 10.2-1 ratio. Most engines that have knock sensors will benefit from higher octane gas.
It may take a couple of tanks for the computer to slowly raise the timing from the lack of knock detected. I would think the programming would be based on averages over a period of time when determining timing to run. Kinda like the fuel trims do. GM's do this, not sure of Chrysler programming.
The ECM will try to run as much timing it can, up to the limit programmed. I doubt any engine running 87 will be at the upper limit. You'll need to log knock retard, with a scanner, to see how much timing is being pulled for each fuel. More timing means more power and better fuel economy.
Not really. Take the Corvette. It is recommended to run 92 octane fuel but you can run regular. It will pull timing and you will lose performance running regular. If you run 100 octane fuel it will add no more timing than was available for 92 octane. You can't take advantage of 100 octane fuel without tuning.

Now there are times when running a higher octane fuel may be called for. Towing or high temps come to mind. But the only way to know for sure is if you are able to data log. If you are on the knock sensor all the time then going to a higher grade a fuel may be called for.
 

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Correct. I filled up the tank in my SRT with regular a couple times in error; I couldn't really tell much difference...maybe because it cranked almost 500 lbs of torque. But it didn't knock or anything.

When I had my 3.8 JKUR loaded to the top with people and stuff for a long trip, I'd fill up with premium just to pull any additional marginal gain I could get.
 

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It's not that complicated if you're a mathematician ... according to this article even mathematicians struggle. But llike I've been saying all along and confirmed in the article, if you hear pinging or slight detonation (same thing) under heavy load, move up one octane level.

Don't need a G.E.D. ... just some common sense.

Compression Ratio and Octane Ratings: What You Need to Know - HowStuffWorks

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