Thin Air

Recently moved from Midwest (700-1000’) to CO mountains (8000’+). Octanes in Midwest are 87, 89, 91. Octanes in mountains 85, 87, 91. In the low country I got 20-22 mpg. Up here, where virtually all my driving is above 8000’, I get 24-27 mpg! Love it. But, other than the thin air, what could explain the improved mileage, lower octanes? And if it is the thin air, why?

My first guess would be that your gas in CO doesn’t have the added ethanol. If that’s not it, then maybe lower wind resistance? The lower octane wouldn’t help, it would hurt slightly, if anything, assuming you’re using the lower-octane gas.

Ethanol is common here (87 octane vs. 89 in the flatlands), including E-85. HOWEVER, I never thought about the lower air resistance, which at 8000’, could be a significant factor. Higher octane prevents pre-ignition, so maybe the lower octane encourages ignition under adverse (thin air) conditions? Inquiring minds?

MTBE was outlawed in CO in 2002. ETBE is apparently still allowed. Of course, ethanol may be used as well.

Even today’s fuel management systems can’t make up for the fact of there being less oxygen at 8,000 feet. Less oxygen, less gas. Maybe that’s why your MPG is better. Be nice if we knew the make model, year of your car & whether it’s turbo-charged.

'07 Toyota Highlander Limited, V6 AWD 17" wheels. Driving style unchanged; uphills cancel the downhills, generally. I really haven’t noticed a reduction in power, which would occur under the “less oxygen – less gas” scenario, but I’m sure that has something to do with it. Odd about the octanes, though – the octane numbers for Premium fuel are the same “down” and “up.”

I do have a camper on a Dodge Sprinter chassis which is a MBZ 3 litre turbo diesel. It get phenomenal mileage, but strangely it doesn’t change in the mountains – might be the turbo keeping things steady (waste gate probably doesn’t open as quickly at altitude).

Thin air = less wind resistance.
Plus, a Highlander is not very aerodynamic, so wind resistance is substantial.

At 35000’ it takes far less energy for a jetliner to cruise at 500mph than at 3000’.

Your car’s computer constantly adjusts fuel flow in order to meet an ideal mix of gas + air for combustion. As you reduce the amount of air your engine is getting, say, by going up in the mountains, your computer automatically reduces the amount of fuel your engine is getting.

Less fuel used = better fuel mileage.

Sounds like it’ll get great mpgs on the moon!

I don’t think less fuel because of less dense air is the reason: let’s take driving on the level, ignoring the wind resistance it would take the same power at 8000 feet as at sea level, so it would use the same amount of gas. What’s different is the wind resistance, allowing better fuel economy.

In the old carburetor days, when we went to higher altitudes, we would advance the timing. Since higher octane fuel has a higher flashpoint to prevent detonation (pinging which occurs when the fuel/air mixture ignites too quickly) this would suggest that in the higher altitudes with less atmospheric pressure, the lower octane fuel won’t ignite as quickly. Therefore the car may use a lower octane fuel. Many automobile owners in the higher altitudes also put smaller jets in the carburetor because less fuel is needed.

I assume that the computers on today’s engines automatically adjust the timing and set the fuel air mixture to accomodate the higher altitude.

I’ll buy it!

Now, what about the octane differences 'twixt low & high, I wonder? I get the power thing, and agree that the same power would be required once the variables are gone. Perhaps lower octane up here is to create a more volatile gasoline/air mixture & better performance – (?)

Computer opens up the throttle in response to thinner air to maintain the same level of power available at sea level. Bigger throttle opening = lower pumping loss.

It wouldn’t take the same power. I’m sure you’ll agree that the mix will be adjusted to deliver less fuel when there’s less air to burn (that is the purpose, after all, of oxygen sensors). At a given speed, a car’s engine is going to be firing the same number of times per mile no matter what altitude it is at. In other words, in 5th gear at 3000 RPM, the pistons will go through a combustion cycle the same number of times over a 1 mile distance no matter what altitude the car is at. That means that even though the engine revolutions are constant, the amount of fuel is not, since there is less fuel being injected into the cylinder with each cycle. Because of that, you’re using less fuel per mile at higher altitudes than at lower.

“What’s different is the wind resistance…”

That could be true. The air density in Denver is about 85% of the air density in Los Angeles if the temperature and humidity are the same.

Thin air, less aerodynamic drag. Less oxygen, less power available for acceleration. Slower acceleration, less fuel burned. It’s like a smaller engine has been installed in your car…

8000 feet + ??? Most people get tired of that in about 2 or 3 years…Just two seasons, winter and getting ready for winter…

Yes, it’ll deliver the appropriate amount of fuel, the same fuel to air ratio. And you’ll have to open up the throttle a bit to get more of that mixture into the engine. It’ll take the same amount of fuel/air mixture (in pounds per hour) to push the Highlander down the road at 8000 feet as does at sea level, except for the effect of thinner air on drag.

I moved up here for the mild Winters. I moved from central Iowa, which gets buried in ultra-frigid Canadian air several times each Winter. Even though Iowa is a lovely state (I escaped there after 60 years in CA), local government and taxation issues (not weather) alienated me, so I ran to CO, West of Colorado Springs.

Life on a mountaintop is fantastic – views ever changing. AND great mileage!

Could be the Highlander has less power here, though I haven’t noticed it. But, then, I’m not in a hurry, either?

I can’t get away from how the roads in the mountains (I have been to the mountains) are usually curvier,have more up and down sections and how here at sea level many roads are straight as an arrow with little to no elevation change. One key to good mileage is a steady application of the gas pedal, this is something you can do at the lower elevations but I find much harder to do in the mountains. Not too many shopping centers at 8000ft just campsites and other undeveloped areas and roads.

Lovely, curving roads; most 2-lane. Huge granite rocks, vast numbers of conifers & Aspens. And I’m only 15 miles from a shopping Center (not unlike in Socal), but 45 minutes to the Springs, for a big city fix (from which I usually flee back to the Mts. in short order). Lots of dirt (decomposed granite) roads. LOW PROPERTY TAXES.

I figure the ups cancel the downs as to mileage, though. Don’t find many speeders here – they don’t last long. You do have to keep an eye out for deer and Elk, especially at night, and even Black Bears (though I’ve only seen their tracks). Tranny does kick down frequently but usually accelerating while going up a hill.

It is gratifying to see the MPG computer showing 27+ MPG, though, after working to get it to read 22 MPG at low elevations. Could be less gas for less air means less power, but I bet the bigger difference is the lower wind resistance in the thin air (my house is at 8800’).

At 8800 feet you can plan on way more snow than you got in Iowa, way way more.