Effect of air temperature on fuel economy?

Discussion in 'Fuel Economy' started by Smile-n-Nod, Jul 24, 2008.

  1. Smile-n-Nod

    Smile-n-Nod Well-Known Member

    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?
  2. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    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.
  3. fixedintime

    fixedintime Well-Known Member

    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.
  4. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    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.
    Last edited: Jul 24, 2008
  5. scottgriz

    scottgriz Well-Known Member

    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.
  6. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    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.
  7. SilentLou

    SilentLou Well-Known Member

    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.
  8. voodoo22

    voodoo22 Cheaper than the bus

    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.
  9. kwj

    kwj I hypermiled this

    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?
  10. ascribe2thelord

    ascribe2thelord Recreational Hypermiler

    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. :eek:

    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.:woot:

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

    lnmcmahan Econoclast

    The hotter the better! I usually run commute segments in the range of 63 - 66. When we had the week of 100F weather, I was pulling 72-75. My only 75 mpg segment came on a really hot day.

  12. pcs0snq

    pcs0snq Well-Known Member

    I look at the Inlet Air on my SG II and I see my avg FE drop 2 points when it's 100F+ over normal mid 80F's in the AM
  13. Blackberry

    Blackberry New Member

    Does anyone know whether a solar or traditional plug engine block warmer would help FE in colder weather? Am thinking it would help the engine warm quicker.

  14. lamebums

    lamebums Member

    My best FE ever was 67 MPG over 11 miles to a buddy's house on I-275 (not counting downhill drives). It was 95 degrees out.

    I never use the A/C unless my left arm is severely sunburned. I bought one of those bead mats you see Iranian cabbies use to reduce the sweating. It really does work and it probably is saving my upholstery, too.
  15. locutus

    locutus MPG Centurion

    Here's about 100 data points of exactly the same route (1 way) in varying temperatures. It hasn't really gotten hot enough around here to see a drop-off above 90 but you can clearly see what cold does to MPG. :(

  16. severach

    severach Member

    Cold temperature keeps MPG low without a WAI. MPG rises as IAT rises until vaporization is sufficient which levels the MPG off. MPG decreases slightly as more of the operating range knocks. When the IAT gets high enough the ECM will dump fuel to keep the engine cool which kills the MPG. The exact numbers are engine dependent since I've seen good running engines with manifolds running at 200*F. Driving to minimize knock and gas additives can extend the ranges a bit.
  17. pcs0snq

    pcs0snq Well-Known Member

    HOW and WHEN did you measure the temperature and the MPG in the scatter graph?
  18. jimepting

    jimepting Well-Known Member

    Very interesting data. I'm curious as to the mechanism of preparing and posting the graph. Could you elaborate a bit on how you did this?
  19. fixedintime

    fixedintime Well-Known Member

    Here is a link to all you ever wanted to know about air density and then some. It even included a calculator so you can see how outside air pressure, temperature, and humidity effect air density.

    With a little playing around air at about 10 degrees is about 20% denser than air at about 95 degrees. I figure that in terms of air resistance to you car as you drive that is about equivalent to increasing the speed by about 20%.

    The effect of changes in humidity seems to rather small.
  20. locutus

    locutus MPG Centurion

    For each drive I take, I record a lot of things - day, time, distance, *what* the trip was (e.g. "House to grocery store"), trip MPG, MFD MPG, some other trip data from my ScanGauge (max/average speed, gallons used, etc) as well as ending temperature as reported by the Prius's MFD. If it's a long trip where temperature could significantly change from the start to the end I'll also record the starting temperature. Trip MPG is computed both from the difference in the MFD (which I only reset between tanks) and from what the ScanGauge reports - I calibrated the Scangauge by making those values match up.

    For "how and when" I collect this data, I just have a little notebook that I record everything in at the end of every trip (while I'm still in the car) and later transfer it to a spreadsheet.

    So I have a HUGE spreadsheet with trip data that looks sort of like this:


    For the graph I did, I just did an AutoFilter with Excel and filtered by a specific trip type. I then copied out the Temp(F) and Trip MPG columns onto a separate worksheet, and made a scatterplot from that.

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