MPG at Altitude (Gas)

[mfifield01]

I just got back from a trip to Pagosa Springs (from Austin). I noticed that my MPG was considerably better at higher altitudes. From Santa Fe to Pagosa, I was seeing ~11 MPG. I manually calculated on the way back and got 10.6 (Pagosa to Santa Fe). I typically get about ~9 around Texas. This is an NA Gas Engine (5.7l Hemi). My only explanation would be gas without ethanol or less wind resistance. Any thoughts?

[Tbos]

We don't really have any high altitudes in the east so I haven't noticed. Switching to synthetic oil made a 2mpg increase for me.


2016 Passport GT 2810BHS, 2016 F350 CC DRW

[B-O-B'03]

I have had the same experience, better mileage outside of TX, with my 5.0 V8 F150 and chalked it up to less ethanol.

I guess it could be elevation or both, not sure.

-Brian

[ken56]

With all the fancy computer controlled fuel injection now days you would think there would be no differences. There is some physics involved that may explain it somewhat. At altitude the air is cold and dense where as in good 'ol Texas its fairly humid and less dense......cold air is heavier than hot air, denser, so it contains more oxygen molecules per pound. Small aircraft have the means to manually adjust the air/fuel mixture at altitude because they are still pretty much old fashioned carbureted engines, otherwise they could run rich to the point that it would flood out and stop running, depending on altitude.

[chuckster57]

Computer controlled engines have the ability to change fuel/air ratio on the fly. In simple terms, OBD II was mandated in 1996 and it is one computer controlling a number of different systems. I can't remember right now how many times a minute it would "adjust" things. C.A.N. was mandated in 2006 (if I remember right), and had a number of computers that communicated with a central computer. If my mind is still working, I believe it made adjustments 60X a second.

With all that in mind, a 1MPG difference may be explained away in any number of ways.

[madmaxmutt]

Originally Posted by mfifield01

I just got back from a trip to Pagosa Springs (from Austin). I noticed that my MPG was considerably better at higher altitudes. From Santa Fe to Pagosa, I was seeing ~11 MPG. I manually calculated on the way back and got 10.6 (Pagosa to Santa Fe). I typically get about ~9 around Texas. This is an NA Gas Engine (5.7l Hemi). My only explanation would be gas without ethanol or less wind resistance. Any thoughts?

With a NA engine, you will experience significantly reduced power at altitude. Therefore, you will burn less fuel. How much less is debatable.

[Gegrad]

I know growing up with my family in the early 90s we got a consistent 2 mpg better when towing at altitude. This phenomenon was observed on multiple trips over several years.

[mtofell]

Originally Posted by Tbos

Switching to synthetic oil made a 2mpg increase for me.

Synthetic oil is typically good for a few % improvement at best. That's some magical oil you got a hold of.

[Johnny's Journey]

Originally Posted by mfifield01

My only explanation would be gas without ethanol or less wind resistance. Any thoughts?

Ethanol burns leaner and requires more. Say yes that's why the increase in mileage. As mentioned about the on board computer. It's sensing a richer burn and is compensating for it.

[mfifield01]

If ethanol is the factor, it reduces gas mileage by about 18% (maybe more).

Another thing that doesn't make sense, is engine load. My engine was working harder to climb to that altitude.

[sourdough]

Originally Posted by mfifield01

If ethanol is the factor, it reduces gas mileage by about 18% (maybe more).

Another thing that doesn't make sense, is engine load. My engine was working harder to climb to that altitude.

I spent about 6 weeks in Pagosa this summer returning about 2 weeks ago. My truck (6.4) does great until I get above about 9500' I guess. It does fine at our mountain house here in NM at 9000' but when traveling over Wolf Creek Pass every couple of days it had a very noticeable drop in power. Figured the ECU would compensate but it must not......hence my consideration of a diesel for my next truck.

[mfifield01]

I didn't notice much loss in power. The highest point on the trip would have been Ski Santa Fe (10,300), but that was without the trailer.

[Outback 325BH]

Higher altitude the air is thinner (less oxygen per volume).

Computer controlled engines automatically adjust the fuel/air mixture so you are always running at stoich -- not lean and not rich.

This means less fuel used at altitude.

However, less fuel and air per volume means less power your engine is producing. This means you will often have to push the pedal down more and/or downshift and/or run your engine at a higher RPM to do any given the task at altitude vs lower altitude.

Turbocharged/supercharged engines don't have this problem because the charger force-feeds the engine and can always maintain the ideal amount of density of air/fuel in the cylinders.

A lot of factors to determine actual mileage ramifications.


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[sourdough]

Originally Posted by mfifield01

I didn't notice much loss in power. The highest point on the trip would have been Ski Santa Fe (10,300), but that was without the trailer.

I think the elev. at Wolf Creek is around 11,300 - 11,500. When puttering around the mountains at that elevation I don't notice anything but when trying to maintain highway speeds up and over the pass it was very noticeable that I had to really "put my foot in it". The grade saps a lot of power but it was more than that.

Note: I was not pulling my trailer back and forth over the pass.

[sourdough]

Originally Posted by Outback 325BH

Higher altitude the air is thinner (less oxygen per volume).

Computer controlled engines automatically adjust the fuel/air mixture so you are always running at stoich -- not lean and not rich.

This means less fuel used at altitude.

However, less fuel and air per volume means less power your engine is producing. This means you will often have to push the pedal down more and/or downshift and/or run your engine at a higher RPM to do any given the task at altitude vs lower altitude.

Turbocharged/supercharged engines don't have this problem because the charger force-feeds the engine and can always maintain the ideal amount of density of air/fuel in the cylinders.

A lot of factors to determine actual mileage ramifications.


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I was aware of this phenomenon but have never experienced it so I had just about decided the computer did enough to make in unnoticeable. On this trip it occurred repeatedly so I'm now positive it's a fact. Didn't have the trailer going back and forth over the pass. I've towed the trailer in elevations to about 10k and didn't really notice anything....could have been the grade. As far as mpg's I'm not sure what it did to it because I wasn't checking.

[Outback 325BH]

Originally Posted by sourdough

I was aware of this phenomenon but have never experienced it so I had just about decided the computer did enough to make in unnoticeable. On this trip it occurred repeatedly so I'm now positive it's a fact. Didn't have the trailer going back and forth over the pass. I've towed the trailer in elevations to about 10k and didn't really notice anything....could have been the grade. As far as mpg's I'm not sure what it did to it because I wasn't checking.

All the computer can do is correct the air/fuel mixture; it can't do anything about less air entering your engine.

Often co-mingled in discussion, the fuel/air mixture and loss of power at altitude are related but different subjects.


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[mfifield01]

The number I have heard is a 3% loss of HP per 1000' of altitude.

[sourdough]

Originally Posted by mfifield01

The number I have heard is a 3% loss of HP per 1000' of altitude.

Yikes!! I hope that isn't accurate. I like to stay in "high" places so I may be in for more problems. It is a big pro in my decision to purchase a diesel but after all the conversations we've had recently about the additives, add ons, extra filters, injectors, etc. etc. etc. and all the issues with them I still can't get on that band wagon yet. Just put a new battery on my truck at the Interstate store and the guy there had a diesel and said he was getting rid of it and going back to gas due to all the "stuff" you had to do and the problems. I know this is off topic so we don't need to get into it on this thread; it's just sort of tangential to the original discussion - and we've discussed it at length in the past.

[madmaxmutt]

Originally Posted by mfifield01

The number I have heard is a 3% loss of HP per 1000' of altitude.

It should be stated in your owner's manual. Buy a forced induction engine and never worry about it

Hmmmmmm. . . It seems some manufacturers do not want to advertise a loss on NA engines at altitude. They must own Apple computers that cannot get a virus.

[Johnny's Journey]

Originally Posted by madmaxmutt

a forced induction engine and never worry about it

Very true