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Old 03-29-2017, 06:08 PM   #1
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Gas mileage, Interesting occurrence!

We Wintered in Florida this year, our first long stay. On the way south we used 87 octane each time and found the further south our mileage per tank dropped more and more. Our truck, a 2014 Dodge Ram 1500 Hemi 5.7 did not seem to be losing power, but I wanted experiment. The last fill in South Carolina I filled using 89 octane and that tankful resulted in a significantly higher mileage. Never one for making assumptions, I wanted to do further analysis. So while in Florida I filled the tank once more, this time with 87 octane and monitored mileage results of the truck on it's own driving an average of city and highway driving. The resultant mileage was lower than what I normally would get per tankful back home. The next time I needed fuel I filled up with 89 octane and found it resulted in about the same mileage as I would get back home on 87 octane.

On the way back north I made a point of filling with 89 octane, until we reached Ohio then since the cost per gallon had risen significantly I switched back to 87 octane, but on this gas my mileage didn't drop to what I experienced on the way south. So here is my question... is there a different makeup of the 87 octane between northern and southerly States? Is there more ethanol in the mix, in the south?
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Old 03-29-2017, 06:41 PM   #2
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The gas is the same but with the increased humidity it causes more resistance, almost like a head wind, and this causes the lower mileage.
Same thing with rain, the increased turbulence from the water droplets on the surface cause a loss in fuel mileage. When the rig dries the mileage goes back up.
My wife says it's physics.
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Old 03-29-2017, 07:06 PM   #3
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The work load of the AC will be higher in warmer climates.

Have you accounted for that ?
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Old 03-29-2017, 07:24 PM   #4
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Hmmmm
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Old 03-29-2017, 07:38 PM   #5
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Southern states use different gas formulations than the north. The percentage of ethanol makes a difference too, less btu's per gallon which means less energy per gallon.
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Old 03-29-2017, 07:54 PM   #6
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We used to have a 2005 Grand Cherokee with a 5.7 Hemi here in Florida. Every time we went to the Midwest or West I would have sworn both mileage and performance increased. So I wasn't crazy after all???
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Old 03-29-2017, 11:46 PM   #7
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Originally Posted by 09 harley View Post
The gas is the same but with the increased humidity it causes more resistance, almost like a head wind, and this causes the lower mileage.
Same thing with rain, the increased turbulence from the water droplets on the surface cause a loss in fuel mileage. When the rig dries the mileage goes back up.
My wife says it's physics.
This is not right. Humid air is actually less dense than dry air, because water vapor is less dense than air. Golfer's know that a ball travels farther on a humid day than it does on a dry one.

Different gasoline formulations are used around the country. Here at elevation (4200 feet on the plains), our mid grade is 87 octane and regular is 85. I hadn't been out of the Rocky Mountain region for several years when I went to Minnesota a couple of years ago for my 50th high school reunion. I was surprised to find that regular was 87 octane. Thinner air has less oxygen, which necessitates changes in fuel mixes. I know that Ohio isn't as high as here, but it's higher than Florida, and the fuel mix is subsequently different.
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Old 03-30-2017, 06:36 AM   #8
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Gasoline does vary quite a bit, depending on location there is winter and summer gasoline, the percentage of ethanol can be 0 to 10 percent and different octane ratings dependent on elevation.

An engine can begin to knock if the octane is too low, so a knock sensor in the engine will detect it and the ignition will be retarded causing increased fuel consumption. Even if an engine is designed for low octane fuel, deposits in the combustion chamber can cause knocking. I suspect that is what you are experiencing.
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Old 03-30-2017, 09:27 AM   #9
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Does your owners manual state the preferred octane? When towing I would use 89.

Computers in the vehicles do reduce timing to compensate for lower octane fuel to help keep detonation to a minimum.

Especially under load.
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Old 03-30-2017, 10:46 AM   #10
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Quote:
Originally Posted by Cqyqte View Post
We Wintered in Florida this year, our first long stay. On the way south we used 87 octane each time and found the further south our mileage per tank dropped more and more. Our truck, a 2014 Dodge Ram 1500 Hemi 5.7 did not seem to be losing power, but I wanted experiment. The last fill in South Carolina I filled using 89 octane and that tankful resulted in a significantly higher mileage. Never one for making assumptions, I wanted to do further analysis. So while in Florida I filled the tank once more, this time with 87 octane and monitored mileage results of the truck on it's own driving an average of city and highway driving. The resultant mileage was lower than what I normally would get per tankful back home. The next time I needed fuel I filled up with 89 octane and found it resulted in about the same mileage as I would get back home on 87 octane.

On the way back north I made a point of filling with 89 octane, until we reached Ohio then since the cost per gallon had risen significantly I switched back to 87 octane, but on this gas my mileage didn't drop to what I experienced on the way south. So here is my question... is there a different makeup of the 87 octane between northern and southerly States? Is there more ethanol in the mix, in the south?

You can not base anything on 1 tank of gas; for the simple reason that one pump may put in more/less gas into your tank then another.

Also, the time of day effects the density of gas. Mornings are colder so the density of gas is more - you get more gs then when it is warmer.

There are other factors e.g. was your truck warmed up to operating temperature when you started the test or was it cold.
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Old 03-30-2017, 12:33 PM   #11
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You can not base anything on 1 tank of gas; for the simple reason that one pump may put in more/less gas into your tank then another.

Also, the time of day effects the density of gas. Mornings are colder so the density of gas is more - you get more gs then when it is warmer.

There are other factors e.g. was your truck warmed up to operating temperature when you started the test or was it cold.
The thermal expansion for liquid gasoline is about 1% from 63° to 85°. Not enough to worry about. That means that for my 36 gallon tank, I'd get .36 more in the tank, which means another 6 miles per tank when not towing (about $.70 worth of gas in an $80 fill up), and utterly insignificant when I am towing (less than 3 miles). It's not an important factor in mileage.
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Old 03-30-2017, 01:07 PM   #12
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The thermal expansion for liquid gasoline is about 1% from 63° to 85°. Not enough to worry about. That means that for my 36 gallon tank, I'd get .36 more in the tank, which means another 6 miles per tank when not towing (about $.70 worth of gas in an $80 fill up), and utterly insignificant when I am towing (less than 3 miles). It's not an important factor in mileage.
You miss the point of my post. It isn't if you worry about the thermal expansion.

The point of my post is that there are many factors that can contribute to a faulty analysis of mpg. I gave some examples of them.
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Old 03-30-2017, 01:42 PM   #13
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The thermal expansion for liquid gasoline is about 1% from 63° to 85°. Not enough to worry about. That means that for my 36 gallon tank, I'd get .36 more in the tank, which means another 6 miles per tank when not towing (about $.70 worth of gas in an $80 fill up), and utterly insignificant when I am towing (less than 3 miles). It's not an important factor in mileage.
That, and gasoline is typically stored in underground tanks where the tempeture is pretty stable at about 60 degrees.
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