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Old 11-25-2008, 12:22 PM   #1 (permalink)
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Does elevation above/below sea level have any bearing on FE?

Quick proviso: Of my two cars, I drive one for mileage, one for fun. This post concerns the fun car

Last summer, I took a long road trip through the southwestern states. At no point was I driving for FE, but there were some portions where topography and traffic conditions lent themselves to fuel efficiency. During these situations, I averaged around 33 mpg.

I drove extremely quickly through Death Valley, enjoying every drop of performance the car had to offer. Filling up in Pahrump, Nevada, I logged my best tank of the trip: 34.7 mpg.

Could Death Valley's below-sea-level (-200 feet) elevation have anything to do with this FE result? I usually average in the 26-to-28 range during spirited driving mixed with highway. It seems incredible that I could run for extended periods at high RPM and still achieve nearly 35 mpg.

For reference, I was driving a 2006 Mazda MX-5 Miata 6MT.

TIA for your thoughts!

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Old 11-25-2008, 01:12 PM   #2 (permalink)
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I think it was the excessive heat that helped your fuel economy there. and hurt your performance, you would have gone faster almost anywhere else.
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Old 11-25-2008, 01:28 PM   #3 (permalink)
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My experience has been just the opposite: I get better mpg around home (elevations from 4500 to 8900 ft) than I do in the near sea level areas around the Bay Area.
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Old 11-25-2008, 01:35 PM   #4 (permalink)
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Hello,

It depends a lot on whether the vehicle has a carburetor or fuel injection.
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Old 11-25-2008, 02:05 PM   #5 (permalink)
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Your good mileage running through Death Valley was the result of the high air temps. The hotter the air, the thinner it is. I live at 1000 feet elevation. To get the same air density that they have at 10,000 feet elevation, the temperature around here would have to get up to 170 degrees F. If you know the air temperature and humidity during your drive through Death Valley, you could look them up in a density altitude chart to see what your air pressure altitude was.

You get better mileage at higher elevations because the air is thinner, which reduces your aerodynamic drag. Also the thinner air reduces the power output of normally aspirated engines and you have to operate at higher throttle settings with reduced pumping losses to get the same power output. Also modern engines automatically reduce the amount of fuel that they use since there is less oxygen in the thinner intake air to mix with the fuel. With older carburated cars you had to adjust the mixture as you went up in elevation to get them to run efficiently, otherwise they would run richer and richer as you climbed into thinner air. Then you had to remember to adjust the mixture back as you descended, otherwise the mixture would get too lean and the engine would become gutless when you floored the accelerator and also risked burning the valves.
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Old 11-25-2008, 02:33 PM   #6 (permalink)
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Very interesting -- Historical weather data indicates that temperatures reached 109°F on the day I passed through. That's quite fascinating.

Thanks for the quick and concise explanation!
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Old 11-25-2008, 10:17 PM   #7 (permalink)
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The high temperature reduced the air density and that reduced your aero drag.
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Old 11-27-2008, 07:46 PM   #8 (permalink)
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Quote:
Originally Posted by basjoos View Post
You get better mileage at higher elevations because the air is thinner, which reduces your aerodynamic drag. Also the thinner air reduces the power output of normally aspirated engines and you have to operate at higher throttle settings with reduced pumping losses to get the same power output.
Bingo. Hot air = higher density altitude. So due to the heat your low altitude was actually providing the FE benefits of higher altitude.
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Old 11-28-2008, 07:47 PM   #9 (permalink)
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A couple of very quick rules of thumb to help out are below.

For every 1 , 000 ft elevation deduct about 3% power.
For very 7 degree C increase in ambient temperature deduct about 1% power.

Not hard and fast but reasonably accurate none the less.

Cheers , Pete.

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