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Old 06-08-2023, 10:18 AM   #1
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MPG this year versus last year same trip

I took an identicle trip this year and compared it to last years trip and found an interesting statistic. I am trying to figure out why.

Trip mileage was within 4 miles of each other (547 v. 551), so that is ruled out.
Weight was same, give or take mabe 10 lbs for food, so that is ruled out.
Motor oil, tire pressure and toad - same.
No generator use.
All fuel loaded to tank fill at each stop.

Why would mpg drop from 8.4 to 7.6 to cause an increase in gallons of 7.1?

I noticed that mpg over three fills went from 6.9 to 7.4 to 8.4 at the last fill. Notice there was improvement at each fuel stop as "old" fuel was dilluted.

My analysis: I think fuel which sits for some time, this trip it sat for over one month, could be thought to have gone "stale". Also, fuel sellers have been given the ok to sell 15% methanol blend which is 5% more than the 10% normally sold-maybe something to do with it.
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Old 06-08-2023, 10:29 AM   #2
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First, there is no methanol in formulated gasoline. Second, all the pumps I visit have a placard stating that the fuel contains up to 10% ethanol.
The wind speed and it's direction could have produced your results.
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Old 06-08-2023, 10:30 AM   #3
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In your case, all the parameters seem to be the same with one exception.....wind conditions. Often you can't feel a slight headwind.

Did you use the same octane on each fuel up?

Did you use AC on both trips?

My coach, which is diesel, jumps all around on mileage. It always averages out in the end from 6.8 to 7.2.
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Old 06-08-2023, 10:32 AM   #4
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It may be the extra ethanol added to fuel. The Stoichiometric Air Fuel Ratio for ethanol is 9 to 1 while gasoline is 14.7 to 1. When we add more ethanol, it takes more fuel to maintain the proper air fuel ratio.
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Old 06-08-2023, 10:37 AM   #5
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Not enough data points and too many variables. While your hypothesis may have merit, there are other factors that can weight in as well. Weather... since you don't have a signature, I'm not sure what you're driving, but pushing a huge non-aerodynamic block 500 miles against a headwind can take a significant toll compared to no headwind, or even better, a 500 mile (distance) tailwind. Fuel... are we talking gas or diesel? Engine maintenance, and so on.

Just one person's opinion and worth exactly what you paid for it.
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Old 06-08-2023, 10:44 AM   #6
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You could be driving slightly different with MPG more on your mind. Are tires going higher pressure due to higher temps.
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Old 06-08-2023, 10:51 AM   #7
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I've got terrible mileage from different fills. Loves being the worst iirc. Could have been pilot. Diesel is hardly close to the same between suppliers and states. Age may be a factor but not in my experience.
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Old 06-08-2023, 11:35 AM   #8
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Three reasons I can think of.

First, wind. Drag from wind is a square function. If you double your speed, the drag from air resistance is 4X. Having a 20 mph headwind one day and a tailwind the next trip will definitely show up in fuel mileage.

Second, Diesel blend- diesel fuel blended with ethanol (I think thatís whatís in it) has less BTU per gallon.

Third, how consistently do you fill your tank? I can get close to 10 gallons in my tank after the auto shutoff. It also depends how level the rig is when fueling.

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Old 06-08-2023, 11:42 AM   #9
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TandW : You are right ethanol.

Dutch Star Don: The trip was identical in terms of dates - Thursday before Memorial Day and return 13 days later. No A/C needed during either trip. Yes mileage does jump around, but I record details on every trip and studied that data for clues.

Vito.a: That is my understanding as well 15% ethanol results in added fuel use versus 10%.

DeWat: Good thoughts. I drive a 14,500 GVW Class C 32 feet towing 3,855 lb Jeep and that combination has remained unchanged for 7 years. You are right by saying there could be more data points-I retired after 43 years as a data analyst, so I get your point. The weather for both years was “about the same” so far as wind was concerned and temperatures. As a weather watcher I feel confident that both trips were about the same.

153stars: I pride myself on driving for precision which means I strive to maximize MPG and avoid unusual traffic conditions.
Shootist: I used the very same suppliers and stations for both trips! Incidentally, I have made this trip for the past 6 years so I am familiar with the traffic at the times of day in which I plan to make the trips.

Thank all of you for your replies. All good questions and comments.
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Old 06-08-2023, 11:53 AM   #10
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Quote:
Originally Posted by Gold Pot View Post
TandW : You are right ethanol.

Dutch Star Don: The trip was identical in terms of dates - Thursday before Memorial Day and return 13 days later. No A/C needed during either trip. Yes mileage does jump around, but I record details on every trip and studied that data for clues.

Vito.a: That is my understanding as well 15% ethanol results in added fuel use versus 10%.

DeWat: Good thoughts. I drive a 14,500 GVW Class C 32 feet towing 3,855 lb Jeep and that combination has remained unchanged for 7 years. You are right by saying there could be more data points-I retired after 43 years as a data analyst, so I get your point. The weather for both years was ďabout the sameĒ so far as wind was concerned and temperatures. As a weather watcher I feel confident that both trips were about the same.

153stars: I pride myself on driving for precision which means I strive to maximize MPG and avoid unusual traffic conditions.
Shootist: I used the very same suppliers and stations for both trips! Incidentally, I have made this trip for the past 6 years so I am familiar with the traffic at the times of day in which I plan to make the trips.

Thank all of you for your replies. All good questions and comments.
Wait, are you talking gasoline? When you called it fuel I thought diesel. I can't see your rig stats if you posted them. You should buy an ethanol test kit off Amazon. Very cheap. And you'll see over 20% when the pump states 15% . They rip us off. Some of my friends can actually smell the difference. No way are you going to get the same mileage, and gas sitting for a year, even with additives is not great.
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Old 06-08-2023, 11:58 AM   #11
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Shootist: yes, gasoline. I am excited to know that test kits are available. Gonna get me some!!

Thanks, Rick
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Old 06-08-2023, 12:42 PM   #12
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Tire pressures and tires exactly the same?
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Old 06-08-2023, 01:58 PM   #13
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Quote:
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Tire pressures and tires exactly the same?
Yes on both items.

Added information - the twin 550 mile trips where run in two segments. The first segment averaged 8.37 mpg over years, but this years average was 7.17 mpg. The second segment averaged 9.11 versus this years average 8.28 mpg.

Both segents showed fuel consumption going down.
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Old 06-10-2023, 12:12 AM   #14
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Quote:
Originally Posted by vito.a View Post
It may be the extra ethanol added to fuel. The Stoichiometric Air Fuel Ratio for ethanol is 9 to 1 while gasoline is 14.7 to 1. When we add more ethanol, it takes more fuel to maintain the proper air fuel ratio.
Ethanol blended fuel tends to burn slightly leaner than ethanol free fuel. Most vehicles on the road today were not designed in mind for 15 percent blend of ethanol. Many vehicle manufactures do not recommend E-15.
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