Thin Air

We experience the same thing when we visit the high altitude mountain areas of New Mexico. My wife’s Honda element usually gets about 22-24 mpg, maybe 26 on the highway.
We got 30.8 mpg in the mountains on our last visit.
Less air to push out of your way when your vehicle moves.
Thin air effectively makes your engine smaller.
Engines don’t need as high an octane rating at high altitude as they do at low altitude.

Hill and dale driving can actually increase your engine’s efficiency. Most engines achieve their maximum efficiency at around 40 to 60 % maximum power. Cruising at a steady 55 mph on a level road demands only about 10 horsepower from your engine. When a car engine is throttled down to 10 horsepower, its thermodynamic efficiency is mediocre. That’s why a lot of gas mileage record setters use a technique called pulse and glide, aka burn and coast. They alternately accelerate at about 50% maximum power and then let the car coast and then repeat. An engine that is making 60 horsepower for 10 seconds and making zero horsepower while turned off for 50 seconds will burn less fuel than the same engine making 10 horsepower continuously.
The hills and dales of mountain driving naturally force the driver to use the engine intermittently, what the hypermilers do on purpose on a flat road.

Here in Tucson we do have a mountain community at 9000ft+ (Mount. Lemon). There is nothing to be gained financialy from living up on the mountain and I never notice any mileage gain driving up there. If you live on the mountain plan on a good 50min from the base to get to your house at the top, you never recover any of the expenses generated by living on the mountain from reduce fuel usage that is noticed at the top, your expenses getting there just eat it up. I am not buying the idea we should all move to mountain tops so we can live in a mpg “nirvana”.

I think you’d be surprised; I was at the cabin in mid-March and there were only a few patches of snow around, while Iowa still had 3’ of snow on the ground, with high berms at the street and sides of the driveway.

More sun in the Mts. Sun melts snow, even in sub-zero temps. Ski resorts are largely on North slopes. When I skied Tahoe, Squaw and Alpine were on East slopes, and sun was their bane. Mammoth has everything, but still has to make snow frequently. Resorts in Little Cottonwood Canyon in Utah are all North slope. Tahoe’s Heavenly Valley was on a North slope, but they still had to add to the Sierra Cement occasionally.

Contrast that to the Mts. West of the 'Springs – storm will blow through, drop some snow, sun comes out & melts it. It’s cold, but less so than the Midwest, with it’s contribution from Canada (how do the Canadians do it?). Even so, I prefer cold to hot & humid. Especially humid, trademarks of the Midwest & South. I think the car(s) prefer it, too.

After a little more thought and reading the other responses (shout out to chunky azian!)
27 is 23% higher than 22MPG.
I think it’s mainly three things:

  1. Less wind resistance.
    jtsanders says the air density is about 85% of sea level,
    That could mean a 15% decrease in wind resistance.

  2. Pumping losses.
    For a given power output and rpm the intake manifold ABSOLUTE pressure will be the same at high or low altitude,
    so there’s less pressure drop across the (more open) throttle at high altitudes.
    There might even be a little less exhaust restriction too.

  3. Lower average speed.
    I’d guess the OP does less cruising above 50mph in the mountains.

I don’t think octane is a factor.

I think you’ve stumbled upon something there – since I began living here full time my driving habits changed in only one respect: there is a lot of “hill & dale” driving. I rarely drive down to the 'Springs, but when I do I get unbelievable mileage; but the drive back ain’t so great. However, once I’m at altitude, along with the lovely curves (roads), is driving up & down hills. So, if I save more going down than I use going back up, it would work out that way.

But I still think the biggest factor is the reduction in wind resistance up here. If Denver, at a mere 5200’, is 85% of L.A., the air should be much less dense more than half again as high as Denver.

I’m still curious about the octane change, though. Premium doesn’t change, staying at 91 in both the flatlands and the mountains. It may be related to compression ratio, but it seems to me the cylinder pressure in thin air would still be less than in the flats and, perhaps, less likely to ping?

You’re right, they could go with a lower-octane premium, but this way they can sell it to folks who believe they absolutely need 91, and, in the case of a turbo-charged car, they’d be right.

Here’ one other possibility - are you finding the improved mpgs by using the car’s computer, or by calculating it yourself? It’s possible the car’s calculation might be thrown off by the altitude change.

I’m using the car’s computer. But I think all that does is measure flow vs. miles driven – shouldn’t be affected by altitude. But here’s an update:

I just drove over to Divide, about 10 miles on curving hills & dales. Except where limited by curves (e.g. 25 mph), the nominal speed limit is 40 MPH – more honored in the breach, I think, than the observance. I’m in no hurry so I usually don’t exceed the speed limit.

HOWEVER, I noticed that coasting down a hill I can sometimes get up to around 50 MPH, which is scrubbed off on the ensuing uphill, so I don’t depress the accelerator on the uphill until I’m near the top and, in a couple of case, never!

I am inclined to conclude, therefore, that a major cause of the great mileage is due, not only to less wind resistance, but from coasting down, and partially up, the hills.

Yowza! I think we’ve got it!

“I’m still curious about the octane change, though. Premium doesn’t change, staying at 91 in both the flatlands and the mountains. It may be related to compression ratio, but it seems to me the cylinder pressure in thin air would still be less than in the flats and, perhaps, less likely to ping?”

Nobody would buy 89 octane “premium”…It’s all marketing and sales volume. The oil companies charge the same PRICE for the 85 octane high-altitude regular as they do for the 89 octane low altitude regular, making it a high profit fuel…