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Mine seems highly variable, going from a low of 29 to a high of 35 or more under similar driving conditions. I just did 315 miles on less than 9 gallons. I thought the pump was cutting off to soon, as they sometimes do, so I ended up filling it to spilling over so it wasn't an underfill. I started at 5800 ft, went to just under 8000, than back to 5800 so there was plenty of up and down. I'm a bit confused. This isn't a very aerodynamic vehicle (not a complaint; that 's not what it's supposed to be) and the only thing I can figure is that sustained driving at speeds much over 65 mpg really kills mileage - even more than rough dirt roads. In my Mazda 65 mph or 85 mph yields an almost imperceptible difference in mpg. Anyone else have this experience - or other ideas? Brand of gas doesn't seem to be it.
 

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You touched on the two factors that impact fuel economy (disregarding driving style): Elevation and speed. Keeping it under 65 will yield higher MPGs. Driving at higher elevations will also yield higher MPGs. Best MPG I ever got in my Tacoma was driving to Bodie SP (got 23 MPG). Drove around Tahoe this weekend (I live near there at 4K ft elevation) doing 45-55 mph and saw my first 33 MPG in the Crosstrek.
 

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Acre, have you tried figuring in that air at 5,800 ft and at 8,000 ft levels is quite a bit different? Seriously ... the rarified air not only makes a difference in your breathing, but also in the gasoline/air mixture being fed into the engine and how it burns. If you are looking to determine mpg you have literally shot yourself in the foot by ensuring the difference in elevation.
 

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Its a small engine being thrown into highly variable altitude, throttle, & even temperature conditions. Its not an issue, seems like a search for a problem that doesn't exist.
 

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Acre, have you tried figuring in that air at 5,800 ft and at 8,000 ft levels is quite a bit different? Seriously ... the rarified air not only makes a difference in your breathing, but also in the gasoline/air mixture being fed into the engine and how it burns. If you are looking to determine mpg you have literally shot yourself in the foot by ensuring the difference in elevation.
This week i brought the crosstrek to 6500 feet mountain, interestingly there is no lack of power feeling, and the x-mode really helps on descending the wet steep broken asphalt. The only trick to practise is to leave the cvt on D and downshift into 1 or 2 to quick overtake any slow vehicle going up to the mountain. Just a pity my son not get used to winding road, so need to slow down to 45 mph while descending where the crosstrek comfortably handling the bends in 55 mph without any brake used. So far mpg stay at 8.9l/100 km according to the meter.
 

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I've tried to address this many times. At higher altitudes, the engine should run leaner (i.e. less power and less fuel used).

Does anyone know if this is what happens? Or does the engine run too rich (and all the problems that entails)?
 

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Air pressure at 5 or 6 thousand feet is quite different than atmospheric pressure at sea level, so your dealing with temperature, O2 level, and pressure. Vehicles have to rely on sensors and computers to figure it all out and map air/fuel ratios.

When I moved from FL to NC my motorcycle ran like $&*#! until I had it properly tuned/mapped now it loves the BRP in fall and winter LOL! (Wife, not so much!)


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AstroKats - I expect fuel injection & computer chips handle it all to maintain optimum performance. But I will quibble a little with the way you phrased it. The engine doesn't run leaner, leaner would imply a lower fuel to air ratio. I suspect the ratio is the same at higher altitudes, however, less air means less gas is being sprayed thru the injectors. And like you said, likely less power being produced.

So then there is the gas mileage conumdrum. Based on the above, you might think gas mileage goes up at altitude because less gas is being sprayed into the injectors at a given pedal pressure. But since less power is being produced, I'd think I'd be pressing the gas pedal harder to get the performance I want. So gas mileage would get worse.
 

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AstroKats - I expect fuel injection & computer chips handle it all to maintain optimum performance. But I will quibble a little with the way you phrased it. The engine doesn't run leaner, leaner would imply a lower fuel to air ratio. I suspect the ratio is the same at higher altitudes, however, less air means less gas is being sprayed thru the injectors. And like you said, likely less power being produced.

So then there is the gas mileage conumdrum. Based on the above, you might think gas mileage goes up at altitude because less gas is being sprayed into the injectors at a given pedal pressure. But since less power is being produced, I'd think I'd be pressing the gas pedal harder to get the performance I want. So gas mileage would get worse.
I've brought this up several times here and it's surprising to me that it's not well understood. Air at altitude and at higher temperatures is "thinner" (i.e. fewer molecules of air and, therefore, fewer molecules of oxygen). So, it requires a "leaner" mixture of fuel and air (i.e. fewer molecules of fuel) for optimal combustion.

"Leaning" is the term that was used when I learned to fly. Maybe there's a different term now but the same chemistry and physics applies. I would expect a car with a clever ECU to use less fuel at high altitudes and high temperatures. I know for a fact that our Crosstrek struggles over 10K ft even in cold weather. What I'm not certain of is how well the ECU manages "thinner" air and higher air temps, which combined is what's referred to as Density Altitude.
 

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I live in Aurora, CO at about 5,500 ft in altitude. I frequently drive to Keystone, CO located at about 9,200 ft after crosssing through the Eisenhower Tunnel, about a 86 mile trip each way. I drive a 2014 Subaru XV Crosstrek Hybrid Touring with just over 23,000 mile.

I track all my actual mileage in a paper log. Going uphill to Keystone and filling up before leaving and after arriving in Keystone I get about 21-22 mpg. Going back to Aurora, which is still alot of uphill driving, I will get in the upper 20's mpg. Averaging about 24-25 mpg for the overall trip.

I generally keep the engine at high tach as I am trying to maintain a speed of about 75mph.

No one that I have talked to has every stated they get better gas mileage at a higher altitude. We have to use a higher octane gasoline to just get the same power that the rest of the country at lower levels.

I am mistified by this theory that higher altitude equals higher mpg. After 50+ years driving small sports cars from Japan to huge American station wagons, I have never seen this theory. This included many trips to the lowlands from North Dakota to Texas and onto the East Coast. My mileage always increased the lower in altitude I went.
 

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If your car needs say 50 hp to maintain 60 mph (~97 km/h) at 8000 ft (~2438 m), you'll have to run a higher rpm to make that 50 hp. When I drove my '08 Matrix out to Portland (Oregon), it couldn't maintain cruise at the slightest incline without downshifting at that elevation. At around 950 ft (~300 m), I can drive up a 6* incline in overdrive and actually accelerate without downshifting (at freeway speeds).

artosa
 

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This is an interesting thread...

Perhaps, because the power is so low at very high elevations, the car is being floored the entire time just to get it going... :D
 

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Well, theoretically, if the ECU adjustment is perfect, a car should achieve better gas mileage at higher elevations. And that is simply because with thinner air, there is less drag. And it makes more difference than you think. At 8000 ft the air is 75% as dense as at sea level. That's why airplanes like to fly high, to save on fuel, and go faster.

The downside is less power at altitude, but only at max throttle. So you will really only notice less power at max acceleration or going up steep hills. Otherwise the car power will feel the same. But another upside is that driving faster at higher altitudes will have less hit to gas mileage than at sea level.

Just some little theoretical facts to consider...
 

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Well, theoretically, if the ECU adjustment is perfect, a car should achieve better gas mileage at higher elevations. And that is simply because with thinner air, there is less drag. And it makes more difference than you think. At 8000 ft the air is 75% as dense as at sea level. That's why airplanes like to fly high, to save on fuel, and go faster.

The downside is less power at altitude, but only at max throttle. So you will really only notice less power at max acceleration or going up steep hills. Otherwise the car power will feel the same. But another upside is that driving faster at higher altitudes will have less hit to gas mileage than at sea level.

Just some little theoretical facts to consider...



From how I understand it, thinner air means the airframe has less drag. But it also means that the engine has less air, and so can combust less fuel. Jet engines get around this by having really large intakes and compressors before the air hits the spot where fuel is added and combustion takes place. Internal combustion engines got around this with some combination of really big air intakes and/or mechanical compressors which forced air into the combustion chamber.

Now if I were to design a naturally aspirated IC engine to run at high altitudes, I would size the air intake to have the mass of air at altitude equal to the needs of the engine where it needs to be to run efficiently. While AstroCats is right that the air is thinner in the ambient environment, that does not mean that a moving vehicle needs to settle for ambient atmospheric pressures. Ram air is a thing. So even though the atmospheric pressure is lower, the mass of air hitting the combustion chamber can be the same, if the air intake is designed to make that happen.
 

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From how I understand it, thinner air means the airframe has less drag. But it also means that the engine has less air, and so can combust less fuel. Jet engines get around this by having really large intakes and compressors before the air hits the spot where fuel is added and combustion takes place. Internal combustion engines got around this with some combination of really big air intakes and/or mechanical compressors which forced air into the combustion chamber.

Now if I were to design a naturally aspirated IC engine to run at high altitudes, I would size the air intake to have the mass of air at altitude equal to the needs of the engine where it needs to be to run efficiently. While AstroCats is right that the air is thinner in the ambient environment, that does not mean that a moving vehicle needs to settle for ambient atmospheric pressures. Ram air is a thing. So even though the atmospheric pressure is lower, the mass of air hitting the combustion chamber can be the same, if the air intake is designed to make that happen.
Not sure what exactly you are saying here. But I am a pilot, and fly a piston non-turbo plane. These engines are designed to cruise at 75% power. So when I get to 8000 ft, I just push in the throttle to wide open (giving me 75% power), and my true airspeed is much faster than at 1000' at 75% power.

So yes, it is all about less drag. Size of my air intake has nothing to do with it. Unless you are turbo charged you can't get more air in with the shape of your intake. It designed to be as efficient as possible at sea level, and that doesn't change at altitude.

My plane is also a floatplane, and can tell you that altitude makes a huge difference in take-off capabilities at higher elevation lakes (even at 4000'). What happens is that your engine makes less power, wings have less lift, propeller has less thrust, and floats have more drag on the water, because it takes more speed to lift off with the thinner air. So everything is working against you.

But once you are airborne you are good. Of course, with the thin air, you are also limited in how high your plane can fly, as it just runs out of enough power to climb and pull it fast enough to stay above stall speed.

But with a car at altitude, you still have plenty enough power for cruising, and with the thinner air, should be less drag, therefore less fuel to go the same speed.
 
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