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Why mileage gets worse in winter

Discussion in 'Gen III 2010+ Prius Fuel Economy' started by cwerdna, Oct 30, 2011.

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  1. cwerdna

    cwerdna Senior Member

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    (10/10/12 edit: Moved running draft text formerly in post #10 to this post. I do welcome feedback about the content or any other links that should be included.)

    Since it's getting colder and there's even snow in some parts of the US already, no doubt people will experience the usual mileage drop during winter/colder weather. This affects all vehicles, not just the Prius. When the weather gets warmer, mileage complaints go down, as mileage rises in warmer weather.

    Here are some links discussing the reasons why:
    Why does mileage drop in winter? — Autoblog Green
    Cold Weather Vehicle Fuel Mileage – Why Winter Fuel Economy Drops – Fuel Mileage Drop in Vehicles
    Car Talk
    http://web.archive.org/web/200912070.../11354696.html
    Why does it get worse MPG in cold weather?

    In addition to factors winter/colder weather that directly affect a car's operation, there may be more delays and more travel time at inefficient speeds due to accidents and weather conditions. Those in cold climates w/o a garage or covered parking may run their engine w/the heater and defrost on high to help melt snow and ice off windshields. Fuel consumption is high at this point since the engine is still in warm up mode and yielding 0 mpg for the entire duration.

    Due to the Prius' typically high baseline mpg figure, the mileage drops may seem worse than they really are. To quote from Car and Driver: Mileage? No, it's Your Gallonage that Really Counts


    Here are some sample calculations to illustrate the above:
    100 miles / 50 mpg = 2 gallons used
    100 miles / 45 mpg = 2.22 gallons used

    100 miles / 20 mpg = 5 gallons used
    100 miles / 18 mpg = 5.56 gallons used

    So, if you had a 20 mpg vehicle that dropped to 18 mpg, you might not even notice it or dismiss it as noise. Yet, the 2 mpg drop in that vehicle actually caused ~2.5x more consumption.

    What can you do?
    Stuff you can do to improve your winter mileage (2010 Prius Grill Blocking strategy | PriusChat has another table of grille blocking suggestions for 2nd and 3rd gen Prius grille blocking)

    Please list your tips for better MPG in the winter. | PriusChat has some good hints although some are 3rd gen (2010+ model year) specific such as the mentions of HSI, likely #6 (the 46 mph that mentioned is likely the higher speed at which the 3rd gen can run in electric only) and blocking the lower grill (on 2nd gen, one should block upper grill first).

    Better heater management (more details at Which one? | Page 2 | PriusChat): if you're using the heater and while stopped, you find that the ICE (internal combustion engine) seems to run or start up "unnecessarily" when the HV battery doesn't appear "low", do this:
    - stop using auto mode on the HVAC system
    - turn the HVAC fan to OFF when you find the ICE seems to be running "unnecessarily" (it will run to provide requested cabin heat if the fan is on and the ICE is too cold, wasting fuel)
    - only turn the fan back on when the ICE must run anyway (e.g. hard enough accelerations or traveling >40 mph on the 2nd gen Prius and >46 mph on the 3rd gen Prius)
    - on 3rd gen (2010+) Priuses, use Eco mode - it reduces the threshold at which the ICE runs to provide cabin heat (when HVAC temp is set to high) from a coolant temp of 145 F to 114 F

    You won't need to the above once the ICE is warmed up enough, but it can cool down enough where it'll need to run to provide cabin heat. Since the Prius has no engine or coolant temperature gauge, there's no way to know how close it is to needing to run unless you have something like a ScanGaugeII : Linear Logic - Home of the ScanGauge or Torque — OBD2 Performance and Diagnostics for your Vehicle.
  2. john1701a

    john1701a Prius Guru

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    Worse in winter...

    The concern will become more and more of an issue as less and less informed consumers buy "Prius hybrids". (Hmm, there's an interesting new phrase.) Fortunately, some of us have been attentive enough already to be collecting data for perspective. Mine has been for comparisons to the previous generations and will soon be to the plug-in model.

    Things like slower travel speeds and accident delays seem to get forgotten entirely. So it would seem as though real-world data is the only way to express expectations, because it accounts for all those factors. Even that's a challenge though, since most people don't actually know the MPG of their current vehicle for each season.

    Winter is world's different in Minnesota than it is in many other parts of the country too. We get extreme cold, which is quite different from the massive quantities of snow and routine ice other warmer northern regions have to deal with.

    Sadly, we've got the new problem of people not understanding how misleading MPG is. That system of measure doesn't inform you of how many gallons of gas were actually consumed, nor does it provide anything with reference to electricity consumption.
    .
    DadofHedgehog likes this.
  3. wjtracy

    wjtracy Senior Member

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    I liked reading the references. However, there may be some conflict in the articles. Autoblog_green seems to stress that winter fuel is to blame. On the other hand, Car Talk and About.com, which I tended to agree with these more, basically downplayed fuel as much lesser importance to friction/other issues. In fact, if you say reduced fuel atomization in cold weather is part of the MPG loss, the winter blend might actually help mitigate that.
  4. efusco

    efusco Troll Slayer Staff Member

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  5. cwerdna

    cwerdna Senior Member

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    Yep, I feel we've definitely seen the more and more of an issue, over the years.

    Good point about the slower travel speeds and accident delays. These articles tend to assume nothing has changed with respect to that. However, some people have commutes where they're stuck in what's effectively a parking lot for most of it, even in good weather.

    Regarding the bolded portion, what's worse is that some folks don't even know the MPG of the current vehicle at all, let alone for each season. And yes, I too ignored and dismissed the arguments for a change away from mpg until I read the "gallonage" article myself and ran the numbers. Perhaps if I'd paid attention/knew at the time, we might've been more successful in calming down folks like Indyking...
  6. cwerdna

    cwerdna Senior Member

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    my idea

    Hmmm.. I was thinking of gathering thoughts here and better sources (if any) and then starting a new post that would be stickied.

    After giving people a a few days to comment/provide better links, I'll start a new post w/a final list of useful links and add a short blurb on heater usage (e.g. if you have no means of monitoring your ICE temperature and you find the ICE running while stopped or idling and the HV battery isn't "low", try turning off the heater). Tony or whoever can then link to that, instead of this.
  7. efusco

    efusco Troll Slayer Staff Member

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    Cwerdna--my suggestion would be to write up a nice comprehensive article that we can put in the knowledge base. You could look at some of the stuff that's in there and then start from scratch, include photos, links, etc.
    Andre V. likes this.
  8. cwerdna

    cwerdna Senior Member

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    Yeah, but I'm too lazy to do that. :embarassed::redface::/

    I think the most I'm willing to spend time on is to put in a cleaned up set of links (of causes and how to partially mitigate the mileage drop), some sample mileage drop gallonage calculations and write up a brief blurb on heater management.

    If someone else is willing to write up a KB article, go for it.
  9. dhanson865

    dhanson865 Expert and Devil's advocate

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  10. cwerdna

    cwerdna Senior Member

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    1 person likes this.
  11. F8L

    F8L Protecting Habitat & AG Lands

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    I know that tire pressure should be checked regularly, especially as the weather changes. Despite this knowledge I have not checked mine in over over a month and temps were in the 90°s.

    So today I figured I would lower the pressure to soften up the ride. I had set the tires at 50/48 psi. I was shocked when I put the pressure gauge on the valve and it read 40psi on the front tires! It was approx. 53° when I checked them at mid day.

    This just goes to show it's not what you know but what you do with that knowledge. Don't be lazy like I was. lol
  12. alfon

    alfon Active Member

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    Winter has arrived here in the northwest. MPG has already dropped about 5 mpg from this time in October when temps were in the 50's to 60's with mostly dry roads.

    MPG were about 52-54 calculated then, now about 47-49 mpg, and when
    the weather gets really bad with standing water on the roadways and temps in the high 30's to low 40's and heavy rain expect 43-45 mpg calculated under these extreme conditions.

    Yes, after 61,000 miles on our Prius no car I have ever owned loves
    summer more than the Prius.....

    al
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  13. F8L

    F8L Protecting Habitat & AG Lands

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    Alfon, I think there are two reasons that the Prius loves summer weather. Well spring and fall for those of us in hotter climates.

    1. The heater and AC use is minimized so the engine will turn off more often.
    2. The battery is more efficient in mild temperatures.

    The other reasons the Prius suffers in winter are not Prius specific as all vehicles take a mpg hit in these conditions. We just notice it more because of the information the Prius provides and because a 10% hit at 50mpg looks larger than a 10% hit at 20mpg. The larger numbers look worse than they are in reality. I realize you know this but I have to post it for newbies. :)
  14. alfon

    alfon Active Member

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    Yes, it does show more at 50 mpg than at 15 or 20 mpg. Also
    here in Oregon we are mandated by State Law to use 10% Ethanol in all gasoline. That sure does not help.

    alfon
  15. 78bonanza

    78bonanza Junior Member

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    Winter gas simply contains more oxygenating compounds . Ethanol, MTBE, etc. These help fuel vaporize at lower temps for easy starts and will allow a cat converter to heat more quickly for emission control. Downside is that these molecules replace some of the power producing molecules and hence mileage decreases. ( a happy note for performance/high compression cars/ engines is octane increases)
    An Internet search will confirm all this so not worry about losing 4 mpg on the first tank of winter gas you get. There is nothing wrong with your Prius.
  16. Calle

    Calle New Member

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    Here's the bottom line, there is not one simple answer as to why mileage suffers in the winter. Anyone who blames it solely on the gas itself, or any other single factor, is simply incorrect. MANY factors come into play here and they are as follows. This comes from someone who used to build engines for a living, has a degree in automotive engineering, and drives a Prius.

    1. The gas you use. Yes, this may or may not have an effect on MPG's. So this one is debatable depending on exactly what type of gas and where you live.

    2a. Temperature. Plain and simple. This is NOT debatable. It is common knowledge (if you know anything about the combustion engine) that colder air is more dense, therefore, in order to maintain an efficient and proper fuel to air ratio, you would need more fuel to burn that air in order to maintain a roughly 14:1 ratio of air to fuel....This factor in and of itself affects EVERY vehicle, not just the Prius.

    2b. Temperature, again. As it relates to density. Colder air is more dense, as we just discovered, and therefore, it requires more WORK (power) to drive through and ultimately costs you more fuel. Again, this affects every vehicle. It is the same reason pitchers in baseball throw faster in the summer, and have more action on their pitches in the winter. To give you an idea of how much air density plays a factor, look at the Colorado Rockies stadium. The air is so thin there in the summer, they had to make the park ALOT bigger than a normal baseball field.

    2c. Temperature-as it relates to water temps and combustion. AGain, common knowledge that a cold engine is much less inefficient than a warm one. That fact being stated, the longer your engine is cold, the more time you spend burning more fuel than you need to. In the summer, if it's 80 degrees outside, well then it doesnt take very long to heat up the car from 80 to 160~...But when its 20 out, you are spending THAT much more time in a cold engine state. On top of that, it is harder to keep the engine warm unlike summer time. Again, this affects every single car on the road.

    3. Another factor is using the heater. By using the heater, the ICE is going to run more in a Prius. But it doesn't stop there....By running the heater, you are essentially removing heat from the engine. So, just because your car is heated up and the initial MPG kill is over, you still have to replace the heat that you are removing constantly!

    4. Snow. Snow obviously affects mileage.

    5. Daylight. In the summer in NY, we have about 15 hours of daylight. In the winter we have about 9. This means more night driving. More night driving means more heat used but even more so, it means you are using your headlights and heater more. This draws anywhere from 90 - 200w out of the traction battery which ultimately needs to be replaced by the ICE. A negligible difference, but still a loss to MPG.

    There are probably a dozen more that I am not thinking of.
    Data Daedalus likes this.
  17. 78bonanza

    78bonanza Junior Member

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    Yes , the above is all true, but may be matter of degree. Notwitstanding 40 years+ of being around aircraft and automobile piston engines I observed a simple divergence:
    In October of this year with temps in the 60s ( Long Island NY)my mileage dropped with one tank of gas from 50.5 mpg to 46.2-47.1 almost immediately. No change in driving patterns, external temperatures of any significance,tire inflation, etc.
    Ran E85 in a sedan I own and dropped 5-6 MPG negating the fuels economics. A far greater reduction than the fuel's porponents quote.
    With the ability to accurately track mileage in our Prius' I am going to be watching during April/May to see if I get as quick a reversal to 50+mpg at each fill up. Should be fun/interesting!
    So many different dynamics taking place with these hybrids that may challenge my previous piston engine experience.
  18. fuzzy1

    fuzzy1 Senior Member

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    While bad for E10, isn't that pretty good for E85?

    Lacking any vehicles rated for E85, I have no first hand experience with it.
  19. 78bonanza

    78bonanza Junior Member

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    Depends, it was break even on cost for me. The net effect would have been simply more gas stops for the same cost. Much better emissions I guess. E85 advocates stated only a2- 3mpg decrease. Wasn't a big demand at this gas station.
  20. SuperchargedMR2

    SuperchargedMR2 Diehard Rams Fan

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    In the NW I notice a good drop when the rainy season hits. Pushing through a lot of standing water takes a toll on mpg on all cars. On top of that my wife likes the heater, who am I to tell her not to use it? :D
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