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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?

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.

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.

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.

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.

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.

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.

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.

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?

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. :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.

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?

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.

Larry

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

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.

D.P.

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.

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. :(

http://www.tripprofile.com/images_tmp/MPGvsTemp2.gif

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.

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. :(

http://www.tripprofile.com/images_tmp/MPGvsTemp2.gif

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

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. :(

http://www.tripprofile.com/images_tmp/MPGvsTemp2.gif

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?

Here is a link (http://wahiduddin.net/calc/density_altitude.htm) 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.

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:

http://tripprofile.com/images_tmp/MPGSampleData.gif

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.

Please see what our CRV does through almost two years in northern Vermont. Look under ma and pa or page down to 27.5 mpg. Winter really takes a toll mostly due to engine oil warm up, engine enrichment, and higher rolling resistance being the primary causes. Jonathan.

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.

Do you have any idea the location of that MFD temp sensor?

The reason I'm interested is the SG II intake inlet temp (throttle body?) in my Fit always seems 5 higher than the ambient.

the IAT sensor is usually on the intake manifold. You can probably find a manual that will tell you exactly which one is the IAT. you can also set your sg to view the IAT and disconnect your sensors one at a time to see when it changes to a ridiculous number that doesnt describe the temp outside or in your engine bay at all.

Any chance we would ever see a graph showing average %epa vs temperature using the data we enter here?

It still doesn't show everything as winds may increase or decrease depending on temperature so temperature becomes more of an indicator than a cause, but it will still be cool to see.

I have a similarly HUGE spreadsheet to Jerad's. I've been recording ambient temperature and mpg, among other things, for 1.5 years now. Here are my results, plotting the average at each temperature point.

As you can see, I don't have very much data for really cold weather, and I'm glad of that!
http://mcguckin.us/files/car/mpg_temp_2008_07.png

I dont have any instrumentation yet...sob... reading this thread, could I assume that blocking of the radiator vents and drawing warmer air from the exhaust manifold could be a good FE thing to do in winter? Water temp and air intake temp meters being desirable

Up to about 80F, I have mine fully blocked. Above that, I start opening it up, to about 1/2 open at 100 or above. For long trips I open it completely.

It makes a big difference in warmup time, so I get to the warm/efficient range sooner.

Here's a good demonstration of what I'm avoiding: http://www.metrompg.com/posts/warm-up-idling.htm

Thanks for all the great work here. I'm feeling a little sheepish as I had removed the ducting from the exhaust manifold to air filter, thinking that 'cooler air is more dense'... and thinking it was put there for snow condition and not our 10 degreeC winters...How wrong I was!! I just whiped down the video store and put it back on, setting the manually control flap to fully heated air to carb, resulting in a super noticeable lift in performance.....I feel like a fool, thinking I had modded an improvement

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.

Would you be able to post some sort of online spreadsheet (Google Documents - Spreadsheet) so we are able to see? I'd like to copy your spreadsheet style and perhaps improve upon it. Thanks.

-Pete

:eek: I'll have to clean it up a bunch before making it public! It's built for my eyes only right now. :o I'll work on it.

:eek: I'll have to clean it up a bunch before making it public! It's built for my eyes only right now. :o I'll work on it.

Thanks a bunch. Do you have any idea where I can look up the daily temperatures for where I live (St. Louis)? I'd like to average out the temperatures per tank of gas...

-Pete

:confused: for some reason my FE has gone down since the air temp. change..... from an avg of 76F up to 84F and maybe a 1-1.5 mpg loss. same conmute.... :(:mad:

I try write it down every day, but when I miss something, I look it up at my local version of this: http://www.weather.gov/climate/index.php?wfo=lsx

Okay, since you brought this up, I've had the chance to drive to work (normally I'm either out of town or at my "home' office).

In the mornings, I can achieve 45, but in the late afternoons, I struggle to achieve 43. But, it is pretty consistent both ways, so I can't tell if it is truely temperature related (mornings in the high 70s/low 80s, afternoons in the 90s), or if it is related to the slight differences in the return highway (curves, hills, road grating differences, road surfaces, etc.)

But, I think I have to agree that when it's over 95, my MPG seems to suffer. This is worth keeping an eye on, so thanks for the additional challenge.

Try it here... hit 106 the past 2 days! My FE is really down.

That's funny. I had my best ever FE mid-day Saturday in Fort Worth when it was about 106, too. Maybe my van works better at higher temps.

On my current tank of gas, when I've been struggling for every MPG, on my trips to and from work (3.3 mi), I get better MPG in the morning (about 80-85F, about 37 MPG) then in the afternoon (100+ for the last few weeks, 30-32MPG). I was thinking heat had something to do with it. However, Sunday afternoon (record high of 107F in DFW), I went to Wal-Mart in the afternoon (6.7 mi) and hit 40MPG. Thursday afternoon, I went to Arlington (16.1 mi, 104F, no A/C) and got 46MPG. It seems the heat has less of an effect the further I travel - maybe it's the fact that the engine is taking more than 3.3 miles to get up to optimal operating conditions.

Would you be able to post some sort of online spreadsheet (Google Documents - Spreadsheet) so we are able to see? I'd like to copy your spreadsheet style and perhaps improve upon it. Thanks.

-Pete

Finally cleaned it up and posted part of my log sheet here: http://www.cleanmpg.com/forums/showthread.php?p=138299#post138299
I have a more crazy and complicated one for daily trip calculations, but that's another ball of wax entirely. Not to be seen in public. :o;)

Interesting thread. I don't see anyone in extreme heat though. I live in Arizona and have been dealing with 110+ the last couple months. Perhaps its not the temperature but the fact AC off is no longer an option on a 115 degree day that my mpgs are much lower in the summer. Also, our winters are much like many of your summers, 60s in the morning, 70s in the day.

Interesting thread. I don't see anyone in extreme heat though. I live in Arizona and have been dealing with 110+ the last couple months. Perhaps its not the temperature but the fact AC off is no longer an option on a 115 degree day that my mpgs are much lower in the summer. Also, our winters are much like many of your summers, 60s in the morning, 70s in the day.

I've been to Phoenix on holidays and my friend and I were tying our golf shoes with our feet resting on the bumper of the rental car when we realized our spikes were going into the bumper because it was so hot. That's extreme in my book.

My father was a crew member on a B24 bomber during WWII. He told me some things from his flying days that have always served me well. He told me that the colder the air was and the more humid it was the better mileage the planes got when they were flying. The big drop in FE in cold weather is caused by a cold engine's demand for fuel until it reaches operating temp. You can see this on the SG when you look at GPH. If the car has set outside on a cool or cold night the car will be gulping fuel for awhile until it warms up. The two best ways to improve the FE are a warm garage or an engine block heater.

In extremely hot weather it stands to reason that the FE might suffer because the air is less dense and, therefore provides less O2.

I'm tempted to try grill blocking this winter but I had a really bad experience nearly 20 years ago. I blocked the grill on my car and it burned a valve. The mechanic said it probably ran too hot due to the block. Has any one else had an adverse experience due to grill blocking?

No trouble. I have a Scangauge, so I know exactly how hot it is. I won't go anywhere near overheating levels.

What is considered overheating these days?

I know my radiator fan comes on at 204* water temp. I try to stay below that, to avoid the extra power draw from the fan. If I stay below that, I know I'm far away from any danger.

how much is too hot in a prius anyway?

Now that the days are cooler here in Western NY, I've got some new questions about the hybrid "modes" in the Prius/Camry/Altima hybrid.

I know that you enter S4 at 163.4F. But, I'm dropping down below 163.4F quite often these days after entering EV Mode. Anybody know what the parameters are if you've gone ABOVE 163.4F, then drop below? Seems like you can still get into EV Mode below that temp, but perhaps with only a certain SOC?

Thanks!
Eph