I just observed that my mileage is considerably better in eastern Colorado (maybe 5000 feet) than Kansas (2000ish), both in flat land high speed driving.
I think it’s likely to be the air density–in CO it’s about half that at sea level. Whaddaya think?
everything is better in Colorado now
Density’s less, but not half:
Mass Air Flow sensor telling the computer what the O2 sensors are seeing, I could see better fuel mileage.
But I bet if you stomped on it, it would be a dog.
Tester
Less air density = less resistance.
Tester made a good point too… without as much air to inhale, there’ll be less power available. Whether it’ll be noticeable or not to you depends on the vehicle and your driving habits.
This is why turbocharged engines don’t deliver anywhere near their EPA numbers. The surest way to increase gas mileage it to cut available power. During WWII people actually drove with a tennis ball under the gas pedal because of gas rationing.
Both of my Lincolns will get about 1.5-2 MPG better in the mountains of CO as compared to the flatlands and lower altitude (1100 ft.) where I live; even when using 85 octane gasoline in CO.
I think the general rule of thumb is a 3% power loss for each 1000 feet of altitude.
When driving in these areas, the headwind-tailwind factor is likely to be responsible for fuel mileage changes…A stiff tailwind can make a noticeable improvement in fuel mileage…
There’s less oxygen available so you have open the throttle wider to get the same power from the engine. A wider throttle opening = lower pumping loss. The engine simply isn’t being choked like it is at lower altitude.
Note that I said driving at constant speed, so the power needed should be the same except for air resistance. Though certainly the throttle opening will be different.
Looking at texases graph, density at 8 km (5000 feet) is about one-third of sea level.
At 3 km it’s about .7 of sea level. So the air density effect is actually larger than I said initially.
Since air resistance is the biggest thing that makes high speed driving gas mileage drop, I think it could be either the throttle effect or the air density/air resistance change.
Nope. 8 km ~ 5 miles ~ 26,000 feet, not 5,000 feet.
At 5,000 feet it’s about 15% or so drop in density.
Air density is roughly proportional to air pressure. Air pressure is 30" Hg (give or take) at sea level, and drops by 1" Hg (give or take) for every 1,000’. Obviously this effect tapers a bit at higher elevations…or the atmosphere would cease to exist at 30,000’…but it’s accurate enough up to 10,000’ or so.
It’s a combination of all those things; they add up.
Wind resistance is a big part of losses at highway speed, so a big chunk of that 15% @ 5000ft.
Pumping loss is substantial.
Just compare coasting down from 60mph in neutral vs top gear.
Yes, pumping losses are different with the throttle closed vs part open, don’t try this: 
On a fuel injected car with a manual trans and throttle cable (no DBW) it’s possible to turn the ignition off in gear at speed and feel the effect of drag vs throttle position as it coasts down.
Again, I’m not recommending anyone try this.
All but the most disciplined driver will use less fuel if less power is available.
Also on cars with knock sensors it’s possible to have more spark advance on average.
On my Matrix I think the knock sensor is used, even running the recommended (87) fuel, when the weather is very hot.
I’ll hear an occasional ping or two when accelerating.
Many engines have a ‘‘sweet spot’’ where they just run smoother , breathe easier and strain less.
Usually pretty hard to find without a lot of experimentation.
Maybe you’ve discovered yours.
the sweet spot is easy to find if you have a bad exhaust leak 
When I visit relatives in western Colorado, about 6,000 feet, sometimes I’ll play a game of golf while I’m there, and one thing I notice straightaway is the ball goes quite a lot further there than in San Jose. I have to changes clubs from what I normally use to avoid over shooting where I’m aiming.
Question: What octane fuel are you using and did you fill up in CO? Fuel in higher altitude areas may be a lower octane and/or formulated differently, which might be the answer as much as the lower air density.
@melott, your power requirement maybe roughly the same, but the manner in which the engine the required power is different depending on altitude. At high altitude the engine can produce power more efficiently