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This one is about MPG. A couple of weeks ago I drove up From Salisbury, MD to Rochester, NY to pick up my daughter for thanksgiving. Temperature at Salisbury was mid 50s when I left, was averaging about 19mpg (display). I noticed that as it got colder, my MPG dropped. Rochester was in mid 30s and I was barely making 17mpg as I approached the city - yeah I was running, but I did the trip in August also and never noticed that kind of difference. I couldn't believe it. Then I noticed same thing on return - MPG got much better as I approached warmer home climate. Yesterday it seemed like we had a heat wave here - 60s - and I had to go to the DC area. MPG was near 20. Returning today, temps were in the low 40s, maybe even lower, and MPG dropped to near 17. Never noticed this before. What is wrong? There's got to be some logical xplanation.
 

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I originally thought of this comment as a joke but then thought that it may be true.
Could it be that the electronics or sensors that make the MPG computer work be influenced by the temperature and the actual MPG is not changing at all or very little?

The colder more dense air flowing through the MAF will cause the computer to adjust the fuel flow to maintain the correct air fuel ratio but I don't think the MPG's should change.
Just a thought----
Mike
 

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Gas "Shrinkage"

One thing that happens when you go directly from a warm place to a cold place is that your gas literally shrinks in the sense that it becomes more dense. This is a real issue with the way they calibrate gas pumps; a gallon a 60 degrees expands at 80 and shrinks at 30. I think the standard gallon is measured at 60F. The more dense gas would cause a vehicle to run richer in low temps but I don't know how much that would be offset by the fact that the air is more dense so you're getting more air and gas. In any event, If you had a five gallon jug of gas that you filled up at 70 degrees and went up a high mountain pass or some such where the temp was thirty or forty degress, you jug wouldn't be full.
 

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If you have ever ridden a motorcycle you would know why. The air is more dense the colder it gets. Riding on a hot day the the air pressure is a lot less on the rider than it is on a cold day... I rode for several years and noticed a huge difference in the air pressure against my body on hot and cold days. Much more drag on cold days.
 

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If you have ever ridden a motorcycle you would know why. The air is more dense the colder it gets. Riding on a hot day the the air pressure is a lot less on the rider than it is on a cold day... I rode for several years and noticed a huge difference in the air pressure against my body on hot and cold days. Much more drag on cold days.
I would think head winds and tail winds would affect you more than air density.
Mike
 

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Those are all factors as well as the change in gas blend. I may have it backwards and have the timing wrong for your area, but about this time in Texas, the blend changes and I believe it is typical to get worse mileage with the winter blend.
 

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I would think head winds and tail winds would affect you more than air density.
Mike
Those are factors too but on a dead calm day you can feel a huge difference in pressure against your body depending weather it is hot or cold on the same road. I used to commute on the same freeway on my motorcycle so I know it was not those other factors.
 

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i think air density would be the biggest factor - i dont care for televised sports but in the few minutes of the van winter Olympics i saw the announcers at the speed-skating were going on about low chance of breaking world records for speed skating at Van as was at sea level. (i.e air density is critical - higher density - slows the human engines - conversely makes your ML need more fuel to maintain the same velocity).

i tried to find some engineering numbers for air density and wind resistance but gave up. i do recall that pee vee equals enn Ar Tea - but clearly that is not enough.

gas blend is not the issue as Dm save his consumption vary in real time as temp dropped.
colder temps would decrease tire air pressure and increase lubrication viscosity - likely smaller effects that air density changes. send government money and we can fund a study...:)
 

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i think air density would be the biggest factor - i dont care for televised sports but in the few minutes of the van winter Olympics i saw the announcers at the speed-skating were going on about low chance of breaking world records for speed skating at Van as was at sea level. (i.e air density is critical - higher density - slows the human engines - conversely makes your ML need more fuel to maintain the same velocity).

i tried to find some engineering numbers for air density and wind resistance but gave up. i do recall that pee vee equals enn Ar Tea - but clearly that is not enough.

gas blend is not the issue as Dm save his consumption vary in real time as temp dropped.
colder temps would decrease tire air pressure and increase lubrication viscosity - likely smaller effects that air density changes. send government money and we can fund a study...:)

That's what I'm talking about. Also they never attempt "land speed" records in the early morning calm because of the air density, another one of my passions...
 

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I experience the same thing when it gets colder. I asked this question awhile back and was told that the engine runs richer air/fuel mixture.
 

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The denser the air the Leaner the mixture because you have more dense air for the same metered fuel. That is why you have to lean out the mixture in a piston driven aircraft engine the higher (less dense) you go.
Mike
 

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One more factor is when the defroster is on sometimes the A/C compressor is on. In the 1998 you can turn off the A/C compressor but some vehicles you can not. With the A/C compressor on it defrosts the whole vehicle much faster that just using heat.
 

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Discussion Starter #13
One more factor is when the defroster is on sometimes the A/C compressor is on. In the 1998 you can turn off the A/C compressor but some vehicles you can not. With the A/C compressor on it defrosts the whole vehicle much faster that just using heat.
Some neat explanations here - the air density explanation seems to make a lot of sense. As for the A/c compressor, mine has the auto climate cntrl so the think kicks in whenever it senses whatever it senses. There was also a significant amount of headwind on my way to ROC and initialy I thought that was the cause. However, I ruled that out after experiencing similar thing on my way back when there wind situation wasn't as bad.
 
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