How Air Temps affect MPG - Ford Mustang Forum
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post #1 of 6 (permalink) Old 01-31-2017 Thread Starter
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How Air Temps affect MPG

So this winter we've had some really crazy up and down weather here in VA. One day it's 30 degrees and 2 days later it's 60 and then back to 30. This has been going on all through December and January. I've been closely watching my gas mileage since I bought my Ecoboost back in Aug. Up till November I was hanging right at 28 MPG average. Then the colder weather hit and it dropped down to nearly 27mpg average. I just chalked this up to the switch to winter gas additives. Recently I've noticed though on days where the air temp is 50+ I'm getting between 27.6 to 28 MPG. If the temps are below 50 I'm seeing decreases somewhere between 27 and 27.5. I use 93 octane from the same gas station each time. The only variance is air temps.
So my question is what exactly is causing this? I am not an expert but my initial thought is colder air being denser causes the computer to adjust the A/F ratio. More or denser air meaning more fuel being injected. Is this thought process correct or do you think it's something else?

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post #2 of 6 (permalink) Old 01-31-2017
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I'm noticing the same thing but I personally think it's effected by more than just temperature...tire pressure changes, driving habits for each tank, gas quality, type of traffic throughout the span of a tank, etc...

Are you adjusting the tire pressure to be the exact same every day? Every 10 degrees is one PSI (up or down). You could be running lower tire pressures which tends to negatively effect MPGs.

There are probably a lot of things contributing to what we are seeing other than the temperature drop. You are seeing such a minimal affect (maybe 1 MPG at the most). Too many variables to consider that I'm not comfortable attributing it to just one of them.

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post #3 of 6 (permalink) Old 01-31-2017
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You're seeing a smaller drop than I would expect.

In winter, lots of things happen. First, fuel retailers switch to "winterized" fuel. It's got additives that help prevent freezing in your tank, but it also has less energy per liter so your fuel economy will drop. Second, the car will run in closed loop mode for a longer period of time because it takes longer to reach operating temperature. This increases fuel usage. Third, the colder temps. increase fluid friction, which robs horsepower, which increases fuel usage. Fourth, cold air is more dense, which pushes upon your car with more force, which increases fuel usage while on the highway.

It's almost impossible to get your best MPGs during winter. I typically see a 2 MPG decrease with my cars and trucks both during winter.

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post #4 of 6 (permalink) Old 01-31-2017
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I have owned a 2012 Ecoboost since new and every winter I notice a drop in mileage.
I have also had the though that the cold, dense air causes a richer mixture to be used.
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post #5 of 6 (permalink) Old 01-31-2017
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It's the fuel in Winter since colder air is more dense and you should actually get much more power in cold air than hot air......fuel being the same!!
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post #6 of 6 (permalink) Old 01-31-2017 Thread Starter
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Thanks. Sounds like for the most part y'all are thinking along the same line as me.
I drive the same route the same time of day every day so there really isn't much variance there.
As I stated in the original post I have been using the same gas from the same station since they switched to the winter additives so that's not it. Plus I can see the variance even off the same tank of gas since the weather around here has been like a rollercoaster.
I would agree with the tire pressure statement but that was the first thing I looked into. In fact when the cold weather first kicked in I put air in my tires. While the pressure may vary from day to day when I first start it seems to be staying constant after a few miles on the road and they heat up so that has been pretty consistent.
I had never given much thought to the friction of the lubricants. Not sure how big of an impact this is though as I always warm up the engine and check the oil temp before I start driving. I'm sure there is a slight impact though.
Good stuff though. I appreciate the feedback.
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