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Is that due to decreased air resistance, or due to preventing the car from getting as much air (thus fuel) effectively lowering the maximum throttle?
Both. Pumping losses in the engine are also lower at altitude. But reduced aerodynamic drag due to lower air density is probably the dominant factor. Air density at 5,000 feet is about 15% lower than at sea level.
 
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Just chiming to give another data point. I've got a 2015 Subaru Outback 3.6R with 63,000 miles.
Around town: 20 mpg (pretty hilly around where I live though).
Highway: 26 mpg (50-65 mph) 24-25 (>65 mph)
 

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Hey, boardsailkick and everybody else!
I just registered in hopes to resolve the same problem. I've got 2018 OB 2.5 and I've put 1600 miles on it, so far, the grand total is 21 MPG! View attachment 489160
According to Fuelly, this is about bottom 5% of reported values for the same year/model. In the city, even driving like grandma, I get 20 MPG. The best I've seen so far is 27 MPG on a flat highway, going about 65 mph. It seems as if I am losing around 5 MPGs both in the city and on the highway.
I have no idea what am I doing wrong, all my previous cars met or exceeded the theoretical MPGs from the manufacturers. I have not serviced it yet, maybe that's the issue. Can oil and filter change increase MPGs by 5?
Addendum: I am in California, so far, no snow or rain, and relatively worm.
Keep in mind that display is generally not close to reality. I log my fill ups and write down both the car's guesstimation and the hand calculated number. I reset 'Trip A' every time I get gas and I always fill the tank, never just add a couple of bucks. My car consistently shows 1.5-2.1 mpg higher than the real calculated value of each tank of fuel.
 

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I use the method of miles divided by gallons used to arrive at an average of 21.5 mpg and a high of 24.5 mpg on long freeway trips with the AC on.
That seems close enough to what it's supposed to get, especially since the 6 cyl gets better fuel economy thaan the 4 cyl at 80 mph.
I'm not sure what the OP was expecting. The display MPG is not accurate for reasons that have been discussed many times before.
 

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Is that due to decreased air resistance, or due to preventing the car from getting as much air (thus fuel) effectively lowering the maximum throttle?
Both, on NA engines, although I suspect the less air argument may be the bulk of it. Less oxygen at altitude is largely overcome with a turbo, depending upon design. I found some articles on “pumping losses” that explain the technical aspects of less air on mpg, but I like the less air/less gas/less hp way of thinking of it.
 

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... the 6 cyl gets better fuel economy thaan the 4 cyl at 80 mph.
Another amazing factoid! (Not gonna' happen.)

If you can provide a link to an objective data source that supports your assertion, I'll be glad to consider it.
 

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Keep in mind that display is generally not close to reality. I log my fill ups and write down both the car's guesstimation and the hand calculated number. I reset 'Trip A' every time I get gas and I always fill the tank, never just add a couple of bucks. My car consistently shows 1.5-2.1 mpg higher than the real calculated value of each tank of fuel.
I tried calculating by hand once and got almost the same number (but it was lower by about 0.5). So, the situation is baffling, you have a 3.6 and get higher MPGs than I do with 2.5 :confused:
What bugs me the most is that I had Mercedes C300 4Matic with 240 HPs that gave me 28 MPGs, and this one has only 175 HPs and takes more fuel..
 

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Another amazing factoid! (Not gonna' happen.)
If you can provide a link to an objective data source that supports you assertion, I'll be glad to consider it.
What would you accept as proof?
 

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What do you have to offer that is not anecdotal?
More importantly, what do you have that is not annecdotal on which to base your review of my data? IOW, what is the basis for your current belief?
 

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^^^ Your assertion, your duty to substantiate:
Honestly I think it is believable that at some higher speed a 6 could be more efficient than a 4 depending on gearing and where each falls at that point in its power and torque curve. At 80mph? With an Outback? I would need to see proof as well but I wouldn't write it off as impossible.

What I find laughable is when people claim to get as good or better mileage at 70 or 80 than they do at 55 to 60mph.
 

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What I find laughable is when people claim to get as good or better mileage at 70 or 80 than they do at 55 to 60mph.
Time for my own anecdote. Of all the cars I've owned, one of them did get slightly better fuel mileage at 70 mph than at 55: my 1969 Porsche 911S. My only explanation for the phenomenon was the unique 911S engine: 2.0 liters, 195 DIN horsepower, 7500 rpm redline, fixed cam lift and timing, mechanical ignition timing, and Bosch mechanical port fuel injection. From curves published in the Porsche FSM, it looked like engine efficiency peaked at about 4000 to 4500 rpm; peak torque ... and peak excitement! ... was in the range of 4500 to 5500 rpm. In 5th gear, the 911S ran ~20 mph per 1000 rpm, so it was turning "only" ~2750 rpm at 55 mph and ~3500 rpm at 70 mph, both still well below peak engine efficiency.

That's my story, and I'm sticking to it. But they don't make cars like that any more.
 

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Honestly I think it is believable that at some higher speed a 6 could be more efficient than a 4 depending on gearing and where each falls at that point in its power and torque curve. At 80mph? With an Outback? I would need to see proof as well but I wouldn't write it off as impossible.
You are on to the key factors. I'm not cruising around town with stop lights at low loads in granny mode where the H4 wins all day. I'm on the highway at an 80 mph average where the H6 has less friction losses, less heat losses, more torque, and half the RPM of the H4. As an example, I drove my 86 Mustang 5.0L from Idaho to Utah and averaged 33 MPG on two successive fill-ups. I averaged 80 MPH with a 3.08 ring gear. Again, I always use actual gallons filled and actual miles driven for my fuel consumption histories. I currently have a 2.5 NA H4, a 2.5 Turbo H4, and a 3.6L H6, so I don't find it impractical to experiment with these concepts.

What I find laughable is when people claim to get as good or better mileage at 70 or 80 than they do at 55 to 60mph.
I haven't made that claim. I've never driven 55 mph for a substancial period of time and that would be right in the H4's bread and butter zone.
 

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Time for my own anecdote. Of all the cars I've owned, one of them did get slightly better fuel mileage at 70 mph than at 55: my 1969 Porsche 911S. My only explanation for the phenomenon was the unique 911S engine: 2.0 liters, 195 DIN horsepower, 7500 rpm redline, fixed cam timing, mechanical ignition timing, and Bosch mechanical port fuel injection. From curves published in the Porsche FSM, it looked like engine efficiency peaked at about 4000 to 4500 rpm; peak torque ... and peak excitement! ... was in the range of 4500 to 5000 rpm. In 5th gear, the 911S ran ~20 mph per 1000 rpm, so it was turning "only" ~2750 rpm at 55 mph and ~3500 rpm at 70 mph, both still well below peak engine efficiency.

That's my story, and I'm sticking to it. But they don't make cars like that any more.
Yep, that could be true. The same thing about the power/torque curve between different engines could apply between certain speeds with the same car as well, even though I ruled that out in my other post. Matching peak power with gearing is critical. I think in most cases though the exponential increase in drag as speeds increase outweigh hitting the optimal point in a vehicle's power/torque curve. Obviously there could be exceptions such as you experienced.
 

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I haven't made that claim. I've never driven 55 mph for a substancial period of time and that would be right in the H4's bread and butter zone.
Yeah, that wasn't intended to be directed at you. I've seen that type of claim both here and other places in the past. It was just another talking point.
 

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I'm on the highway at an 80 mph average where the H6 has less friction losses [vs. the H4] ...
How so?

... less heat losses ...
How so?

... more torque ...
More torque available? Without question.

... and half the RPM of the H4.
Demonstrably not true. Hyperbole, maybe?

In top "gear" ratio, the H4 Outback runs 32.2-32.5 mph per 1000 rpm (depending on OE wheels/tires), while the H6 runs 37.0 mph per 1000 rpm ... about a 12% difference. (Source: 2015 Legacy/Outback FSM, OEM tire "revs per mile" data) Thus, cruising at 80 mph, an H4 Outback would be turning ~2470 rpm, while an H6 would be turning ~2160 rpm ... hardly "half the RPM."
 
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As an example, I drove my 86 Mustang 5.0L from Idaho to Utah and averaged 33 MPG on two successive fill-ups. I averaged 80 MPH with a 3.08 ring gear.
That's incredible!






Who would have expected that an '86 Mustang could make it all the way from Idaho to Utah? :devilish:
 

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That's incredible!

Who would have expected that an '86 Mustang could make it all the way from Idaho to Utah? :devilish:
Having done two head gaskets, two engine replacements and a CVT replacement on the Subaru, I find the Ford Mustang refreshingly reliable...

With the 86 Mustang, it turned out that I was burning lean, a few weeks after the Idaho trip, I discovered an intake-side air leak. It's been too many years to remember what exactly it was, but it wasn't so lean that it pinged and the Mustang doesn't have a knock sensor. I had something similar happen in my 1988 Pontiac Lemans (Daewoo 1.6L). I was getting 38 MPG and the car was only supposed to get 26. It would occaisionally stall while I was cruising and I learned to put it in neutral and refire it. I pulled the dist cap and the stator fell out along with the electronic ignition parts. The stator was loose on the dist shaft (should have been staked) so there was some sort of motion there that was probably advancing the timing. It would ping during acceleration, but not at cruise. I logged 41 mpg on a highway trip. All gone when I replaced the broken dist. Now I get 22-26 MPG.
 
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