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So a few months ago I moved from Colorado (~6400ft above sea level) to Florida (~10ft above sea level). Immediately after I moved the fuel efficiency in my car dropped significantly. Previously I was getting 27-28mpg, and now I'm getting 22-23mpg.

I would have expected the opposite as now the engine gets more oxygen so I would think it runs better. And indeed I've seen several sites on the internet supporting this. Unfortunately I've also seen several sites claiming the opposite.

Which is it supposed to be? And why?

If I am supposed to be getting better fuel economy at lower altitude, any ideas what could be wrong?
I'm measuring fuel economy by how much fuel the car takes at the pump & my odometer. I've tried resetting my ECU, which had no effect. My driving has changed a little, but is more freeway driving now.

Car is a 2002 Mitsubishi Eclipse Spyder. Its a 6-cylinder manual transmission. Uses premium fuel (89 in Colorado, 93 in Florida).

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Well lets assume the computer makes adjustments to keep a consistent fuel/air ratio...then what happens when you add more air? You have to add more fuel. Of course it's not all that simple but it's what my instincts point me to :P –  Rig Feb 28 '12 at 3:01
    
@Rig More fuel and more air means more power, thus I shouldnt have to use as much gas to obtain the same speed. –  Patrick Feb 28 '12 at 3:15
    
Do you know that your pedal application has been changed? –  Rig Feb 28 '12 at 4:35
    
@Rig you mean do I notice it? Its hard to tell. Perhaps, but it could simply just be my imagination. –  Patrick Feb 28 '12 at 10:41
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Do they use methanol blend gas in Colorado and in Florida.Ihave noticed that the methanol blend can reduce mpg by as much as 10%.Iknow in Massachusetts our fuel is similar to California fuel.Idont knowif it is the same in florida –  mikes Mar 2 '12 at 13:00

3 Answers 3

What may be more important is air temperature - the much warmer air you will be getting in Florida will not give you as much power. Cold air is much more efficient for a car engine to use as it is denser.

I know I get a noticeable power increase during a cold winter here, and there are power boost kits some of my friends use which simply spray cooling air over the intercooler to keep the engine cooler.

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No go on this theory too. The summer in colorado is about the same temperature as it is in florida right now (though still a little cooler in florida right now). And I got barely any noticeable difference in efficiency between summer and winter in colorado (small enough to be within margin of error, <1mpg). –  Patrick Feb 28 '12 at 10:35
    
hmmm - interesting. I have to admit the link between power and mpg may not be direct. I get a dramatic difference in power but didn't check mpg at the same time. Your ECU may do something relatively intelligent –  Rory Alsop Feb 28 '12 at 10:44
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I'm beginning to think this question is not answerable. Too many variables are running around loose. If we were talking about a car strapped to a dyno running the same throttle settings, then we might be able to compare mileage. –  Bob Cross Feb 29 '12 at 0:41

I can think of one mechanical/thermodynamic reason why you would see some of loss of mileage, although I suspect that a change in daily driving routine and possibly fuel (ethanol content) are together a bigger bigger factor.

Yes, higher altitude will decrease you engine's power output at wide open throttle. 3% per 1000feet is a figure that is thrown around, however I don't have a proper source for that. This doesn't mean that you'll loose efficiency at higher altitude as most of time the engine isn't producing peak power but is being throttled to stop it from drawing in enough air to make peak power.

Throttling results in pumping losses. The piston is doing work in order to draw fresh charge into the cylinder at manifold pressure (vacuum) against crankcase pressure (at atmospheric pressure). At higher altitudes atmospheric pressure is lower and therefore less throttling is required resulting in lower pumping losses.

I found this website with a good explanation of pumping losses, but beware; it contains at least two P-V diagrams!

http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine

A break-down of emissions is available here:

http://www.fueleconomy.gov/feg/atv.shtml

That page claims that pumping losses account for only ~4% of vehicle losses, however this seems quite optimistic to me. Keep in mind that your 3.0L V6 is quite large by world standards. Those 4% pumping losses might have been measured on a smaller engine, I'll look into what vehicle they did their tests on, it's been bothering me for a while.

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Humid air is less dense than dry air.

Imagine a cubic foot of perfectly dry air. It contains about 78% nitrogen molecules, which each have a molecular weight of 28 (2 atoms with atomic weight 14) . Another 21% of the air is oxygen, with each molecule having a molecular weight of 32 (2 stoms with atomic weight 16). The final one percent is a mixture of other gases, which we won't worry about.

Molecules are free to move in and out of our cubic foot of air. What Avogadro discovered leads us to conclude that if we added water vapor molecules to our cubic foot of air, some of the nitrogen and oxygen molecules would leave — remember, the total number of molecules in our cubic foot of air stays the same.

The water molecules, which replace nitrogen or oxygen, have a molecular weight of 18. (One oxygen atom with atomic weight of 16, and two hudrogen atoms each with atomic weight of 1). This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic foot; that is, it's density decreases.

Source

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This is definitely some food for thought. But wouldnt it somewhat balance out then? Air in Colorado is less dense (due to altitude), air in Florida is more dense, but your humidity idea negates that. And if youre claiming my performance drop is purely over density, then less dense air weighs less (per ft^3), and so Florida should have lower air pressure, but it doesnt. I would also expect my MPG to change based on whether its humid or not out (which I've never had happen). Not saying your idea is bad or wrong, I just would like to make sense of these points :-) –  Patrick Feb 29 '12 at 6:08
    
Not really. You must also remember Florida is also hotter than Hades, so you might run AC less. Hot weather affects the molecules of both air and fuel in a violent way, as the temperature rises, the molecules collide with each other more. So while the air is the "same" density, the cold calms the molecules in Colorado. This means that the air is still when you are trying to suck it into your engine, versus violently colliding molecules trying to be compressed in Florida. –  FossilizedCarlos Mar 1 '12 at 4:32
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As a point of reference, humidity isn't enough of a factor to even make it into the operations manuals for light airplanes (unlike the other engine output factors such as altitude and temperature). It's indeed a known factor, just not big enough to even be worthwhile mentioning in the performance data... –  Brian Knoblauch Mar 1 '12 at 19:11

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