MPG vs Temperature - Some Data

sorry have to dispute your conclusions. my data shows that the warmer it is, the better the mileage with the only variation being whether air is used or not. also you use average daily temperatures how do you calculate that??

i have to assume you are either extremely warm blooded or you are using weather channel data. that makes you conclusions extremely rough. looking at your data makes me think you are doing something at 70º to lower your mileage so i have to believe you dont turn on the air at 70º but more like 85º but the daily average after considering the overnight temps may lower the average daily temps.

i am tracking mileage by temps based on the car after being on the road 10 mins (i notice that it usually takes about that long to level off to a realistic level the actually time really depending on the speed of the car)

i have noticed a nearly 2-3 mpg difference from the early morning summer drives at 55-65º and the early evening commutes at 80-85º so my data directly opposes yours

 

I think as long as you specify the data was taken with A/C on (or at least that the cabin temperature was kept constant), that will help. I agree, you don't have a strict MPG-vs-temp chart, but you probably do have a 'real world' version of it. People don't run without A/C at the temps you describe, so this may reflect what they will see, regardless of the reason for seeing it. The hotter it gets the higher the A/C drag, and the lower the efficiency of the batteries.

Of course the real trick here is to try and keep all the other variables constant (wind, rain, route, speed, etc) while only the temperature changes. I know you probably didn't have much control over these other variables, which is why you are averaging so many data points.

Interesting chart.

 

ok Paul i DEFINITELY wouldnt test mileage without air conditioning either in your case...wow...

i actually have several hundred data points and was never tracking temperatures but the difference in mileage for round trips with neglible wind and elevation changes makes it obvious that temps do matter and until we get above normal ICE operating temps, i think the mileage will improve simply because the Prius gets to running temps faster which shortens to inefficient warmup cycle. i realized this mostly by accident because i worked 10-11 hour shifts all summer, i would leave in the morning for work and come home in early evening so i missed the hot part of the day.(that is why i have a lot of data with no air) sure the car was still warm but i even had the windows down and still improved my mileage

i just installed my canview from efier and its very much like it was when i first got my Prius. now i have a whole set of data sources and i most definitely intend to check out several different things.

bit off topic, but in just one hour of having it (first thing i HAD to do was show it off) and i already have seen that even the slightest brake pressure creates much more regen and brake lights dont use enough to register unless you push hard enough for the ABS pump to start running. (it uses about .1-.2 kw)

but i digress. i will be tracking (well attempting to anyways) how fast the ICE warms up at various driving conditions etc because of the warmup cycle points made above. cant wait

 

It is hard to isolate outside temps from other variables that we know affect mileage, such as first warm-up of the day, changing routes, road conditions, wind, and different gasonline formulations.

For me the overwhelming difference I have noticed is the time it takes for the engine to warm up to it's optimal operating state. If I can drive for an hour at a time, my MPG stays in the 60's even in freezing weather. Shorter trips, and temps below 60 F leave me hoping for 55 MPG. When it is below 32F and the trip is short, 50 MPG looks darn good.

 

Keep in mind there's more than one temperature to worry about.

Each has it's own effects! Assuming you start out in the morning and it was cold the night before, usually it is warmer when you leave.

So temperature of the battery may be closer to the night time low.
Engine may be closer to the current temperature, but with a boost from the thermos,
which will be affected by the night time low.

Then under way the engine warms up fastest, but the battery warms more slowly.
Max economy only happens when both are warm.

Also the catalytic converter has to be warm, and I've seen delays in the good economy operation after engine fully warm that may be due to that still warming up.

So far with the can vue I've seen rapid warm up of the engine due to my block heater!
So operating temperature of 170 is achieved at about 2 miles. This at 40 deg F ambient.
But the battery only goes up like 10 degrees on my whole commute of 12 miles, so never gets to normal temperatures. I see the effect of this in higher engine revs for all conditions as well as less battery mode.

 

Humidity can have a large effect on air conditioning load and perhaps engine performance. Air conditioning load in 90 degree weather will likely be different in Houston than in Phoenix. I don't know if this really applies to the issue here, but thought I would mention it.

 

I agree the data is what the data is. I wonder however at the effect of using average daily temps. Previous posts pointed out the differences between a cold engine in the am and a warmer engine late in the day. I may just be that the sample is too small. I wonder if you had enough sample if the other variables would tend to average out. I expect we need more people collecting data and I think we need data by the trip. I wish I had paid more attention in statistics.

 

the only way to track it is to take the average temps during the trip in question.

my take. record tank mileage by trip odometer (it measures .1) and mpg average on MFD. observe temps (may or may not change dramatically as you start moving. in the morning it rarely changes much. in the afternoon, it frequently drops several degrees after you start driving)

i observed higher mileage when warmer. no air on any of these. (if i used air, the results were discarded...should have kept them too) special attention to RT's to reduce the effect of weight of gasoline.

now most trips were short, 8.8 miles low speed, 25 mpg 10%, 40 mpg 25% and 35 mpg 65%. even with the shortness of the trip, the mpg average still went up but only consistsntly in the evening trips home which happened just after the warmest part of the day. (about 7-8 pm. sun doesnt set untill as late as 10 pm here) had drivers side window down 85% of the time on way home, only once in the morning. (there was no mpg change i could detect with window up or down. this was tested several times using a reference point after car was warmed up)

**other observations i saw. i am fairly certain that the weight of gasoline does play a part but will need to observe that again to make sure. unfortunately, it will be difficult if using any kind of climate control because of the effects of overall economy.

now because of the bladder system on the Prius, the usual pitfalls of doing this does not apply,(excessive water condesation) so i will devote at least a few months to checking on mileage obtained by only filling the tank partially. i want to make the observation in 3 groups of about a 1000 miles each to see if i can see any trend.

 

That's great data. Congrats on being so disciplined to capture so much data.

Regarding the drop in FE at higher temps, I would imagine that as ambient temp goes up, the radiator fan comes on more frequently, so that's another drain on the system, other than ac.

 

I also get high temps in MS. What I have been watching is the relation between the Prius MFD outside air temp and my Scangauge II intake air termp. they stay within 5 degrees of each other until the 80s then diverge rapidly, by 95 I have 15 degree warmer intake air (after 10 minute at 61 MPH) than outside air. (when the ICE is not running it may be 135 under the hood)

I get 40 MPG in winter, even with upper grill blocking, 47 MPG in spring and fall, and 42 MPG in summer. Not being from MS, I use a lot of A/C to stay alive in summer. Some of it is heating the air under my hood.

 

Great data set Paul.

So it looks like about 75F (23 to 24 C) is about optimal. That rings true with my experience, I get my best FE in autumn and spring when the temperatures are around 70 to 80F. My FE drops a little bit in summer and a little bit in winter. Nice to see that so clearly shown on your graph.