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Fuel Economy Discuss how to achieve better fuel economy.

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Effect of air temperature on fuel economy?

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Old 07-24-2008, 08:48 AM
Smile-n-Nod Smile-n-Nod is offline
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Effect of air temperature on fuel economy?

How much does air temperature affect FE? I usually get slightly better mileage (26 mpg vs. 24 mpg) on my morning commute at 80 degrees than the return trip in the evening at nearly 100 degrees. However, my morning commute usually has a slight tailwind and my evening commute usually has a noticeable headwind.. If wind were not a consideration, how much should my mileage vary with air temperature?
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Old 07-24-2008, 08:56 AM
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Re: Effect of air temperature on fuel economy?

I find that as well. The best temperatures for my and my car are 75-85F. It starts to taper off above that. Below that, it doesn't just taper, it falls off. There's another huge drop when you get below 40.
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Old 07-24-2008, 11:41 AM
fixedintime fixedintime is offline
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Re: Effect of air temperature on fuel economy?

I would not rely on using the same route in two different directions to evaluate the effects of temperature on FE. There are just too many other variable that come into play. The light timing, the turns you make, the way the hill come into play, and how heavy the traffic is, are just a few variables.
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Old 07-24-2008, 01:14 PM
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PaleMelanesian PaleMelanesian is offline
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Re: Effect of air temperature on fuel economy?

I've been tracking this for 1 1/2 years, recording every single trip in the car. I have 683 data points.

75-85 degrees is best.
above 85 is next best.
below 75 is third best, tapering down to about 45.
below 45 is simply bad.
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100 mpg commute 90.2 mpg / 1191 mi tank

Last edited by PaleMelanesian : 07-24-2008 at 01:23 PM.
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Old 07-24-2008, 02:32 PM
scottgriz scottgriz is offline
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Re: Effect of air temperature on fuel economy?

I have been tracking air temp and mileage over the past 3 weeks. I only have my data on paper right now, but a quick look at it clearly shows a correlation between air temp and FE. Some of my best days were in the high 80's. As for what temp is best, I will defer to Andrew with his 683 data points. Impressive.
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Old 07-24-2008, 02:58 PM
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Re: Effect of air temperature on fuel economy?

Of course, it's all specific to my car. It does include summer, winter, and now another summer in progress. The high temperature (90+) numbers are still suspect, but I'll have a chart to post soon.
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88.3 mpg commute 68.3 mpg / 801 mi tank
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Old 07-24-2008, 09:19 PM
SilentLou SilentLou is offline
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Re: Effect of air temperature on fuel economy?

I have seen a big dropoff in power and fe both around 85-90F and usually over 100F, but have not seen this since early summer. Humidity may be the other factor, higher humidity means less dense air but may have other effects besides air density.

I noticed one time when the dropoff occurred that the sgII indicated the engine was running OPEN LOOP instead of CLOSED LOOP. I believe OPEN LOOP means the O2 sensor is no longer used for feedback to control the fuel mixture. If that is the case, that could explain the dropoff.
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Old 07-25-2008, 08:38 AM
voodoo22 voodoo22 is offline
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Re: Effect of air temperature on fuel economy?

Our Yaris seems to use less gas the warmer the temperature, but even just below 25 celsius it seems to start using a lot more gas.
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Old 07-25-2008, 09:01 AM
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Re: Effect of air temperature on fuel economy?

Interesting. I've been keeping a close eye on my coolant temperature, striving to keep it as high as I can without the coolant fan operating. I tend to get my best MPGs at this temperature. However, I've not monitored air temperature. It would seem these should have a positive correlation for my car, Yes?
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Old 07-25-2008, 06:41 PM
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Re: Effect of air temperature on fuel economy?

Quote:
Originally Posted by Smile-n-Nod View Post
How much does air temperature affect FE? I usually get slightly better mileage (26 mpg vs. 24 mpg) on my morning commute at 80 degrees than the return trip in the evening at nearly 100 degrees. However, my morning commute usually has a slight tailwind and my evening commute usually has a noticeable headwind.. If wind were not a consideration, how much should my mileage vary with air temperature?
For me the best temperature is about 80 degrees ... comfortably warm. Suddenly I am reminded of a Pink Floyd song.

Above 95 degrees I start getting an itch in my back that tells me to turn on the A/C. Over 100 degrees and I can't go more than a few miles without turning the A/C on. It's just unbearable. You can try drinking a lot of cold water to postpone turning on the A/C, but you still have to turn it on after 10 miles or so. Fortunately my commute is only about 8 miles, but on really hot days I still resort to using the A/C. August is coming.

Even when you turn off the compressor for going up hills and other high engine load situations, the A/C taxes your FE. Good news though, I just got a front end alignment and repaired a loose wheel ... that should mean 30+ mpg for the rest of the summer, even with A/C.

I think I may break 33 mpg again with this next tank.
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