fuel economy vs climate control vs fogged windows

Discussion in 'Gen 5 Prius Fuel Economy & Prime EV Range' started by winnertakesteve, Jan 4, 2026 at 4:46 PM.

  1. winnertakesteve

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    hey all,

    first off, during the warmer months, my 2025 prius was doing pretty solid fuel economy numbers. coming from a 2012 prius c, the new car generally matched or exceeded summer driving mpg quite easily. happy customer there.

    now winter has arrived, and as expected, fuel economy has taken a hit. i know the usual culprits: colder temps, snow tires/snowy conditions and increased climate control use all factor in, especially on shorter drives.

    what i have noticed, especially compared to my outgoing prius, is how much larger of an impact the climate controls play in this newer vehicle.

    my prius c still probably got about 5.5 to 6L/100km in the winter, and in fact it seemed to prefer having the climate set around 20-22ºC for getting those numbers, vs trying to drive the car cold.

    by contrast, the new car struggles to even hit 8-9L/100km even with the climate controls set to eco, and limiting it to 19-20ºC. it doesn't seem to improve much over longer drives, either. it just seems to do noticeably worse than the prius c, all other factors being equal.

    what does make a huge difference is turning off the heat. even on short 15-20 minute drives off a cold start, the engine will come to temperature and the new car will go back to being a 5 to 5.5L/100km performer quite rapidly.

    i'm used to dressing for the weather, so i wouldn't mind this, except the car also fogs up extremely quickly without any heat. so it seems i need to have some form of heat running almost constantly to avoid the fogging issues, and consequently worse fuel economy than its 13 year old predecessor (and still a colder cabin, too!)

    is there any trick i'm overlooking? ultimately it is what it is, but i'd be lying to say i'm not a bit disappointed especially compared to the c.
     
  2. Trollbait

    Trollbait It's a D&D thing

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    It's a bigger car. The EPA specs has the gen5 at 7 more cubic feet of cabin air heat, plus more cold surfaces to heat up. Then the larger windows mean more heat loss.

    Perhaps those window films for keeping heat out will help keep heat in.
     
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  3. BiomedO1

    BiomedO1 Senior Member

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    Larger, heavier, and larger engine car. Larger car meaning larger surface area, mass to heat up. Larger displacement engine burns more fuel and has to move a larger mass. Your pretty much trying to compare apples to oranges. The Prius C had a much smaller traction pack.
    You can try to preheat the car while it's plugged-in to off-set some of this, but the engine will kick ON below a certain temperature anyways. As far as defogging the cabin, you can stop breathing or turn AC on, set cabin air to recirculate, and set your cabin temperature. The AC system will dehumidify your cabin to prevent fogging. If it's external fogging, you can only crank up the heat on your window defogger and use your wiper blades.
     
  4. VelvetFoot

    VelvetFoot Active Member

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    It makes you wonder why they have the "OFF" button, right? LOL. Perhaps it helps with cycling the system on and off manually, lol.

    I don't have a PHEV, but I've noticed that on my '26 HEV. Push the OFF button, and sometimes goes immediately into EV mode.

    Remember, the previous poster is from Cali.

    Snow on boots doesn't help with the fogging.

    However 9 l/100km comes out to 26 mpg (us). That really sucks.

    I'm also blaming the fuel winter mix. I don't have snows though, yet, lol.

    I was bitching about a 53mpg tank that included a 150 mile trip south (300 miles total) from Albany, NY on Christmas Day. Is Toronto that much colder than Albany? Other tanks locally are 55 or so, as I recall.

    I also have the hvac system (only) set to "E" as well (have to read the manual to see what that really means, lol) and temp at 70.

    Does the PHEV come into play? You're not driving it on battery are you?

    It probably runs the engine at times just to heat up the cabin.

    My TDI (diesel) Beetle was similar. It took a long time to warm up. People put cardboard on the grille, but I understand our cars have automatic louvers.

    On full tank refills, locally driven, I've gotten (actual):
    12/11 54.5
    12/18 54.6
    12/24 56.8
    12/29 52.8
     
  5. bisco

    bisco cookie crumbler

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    it is surprising
     
  6. VelvetFoot

    VelvetFoot Active Member

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    @winnertakesteve : do you have it on Auto? I just have the fan on low and temp at 70f.
    edit: I also read the manual on eco mode/heating-seems to control engine speed? Hard to tell from manual for both.
     
    #6 VelvetFoot, Jan 4, 2026 at 8:06 PM
    Last edited: Jan 4, 2026 at 8:19 PM
  7. Trollbait

    Trollbait It's a D&D thing

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    Another possible factor is the hybrid vs. PHEV battery differences. The Prius c had the battery in the cabin like Toyota's other hybrids. That and its small size lets it warm up quickly, and it doesn't take much to keep it at the preferred temperature for efficient operation. The gen5 PHV moved the battery to outside the cabin space. It doesn't get the benefit of cabin heating helping with warming up its much larger mass.
     
  8. bisco

    bisco cookie crumbler

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    i know the heat pump is supposed to be efficient, but could it be part of the issue?
     
  9. VelvetFoot

    VelvetFoot Active Member

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    That's what the PHEV uses for cabin heat?
     
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  10. bisco

    bisco cookie crumbler

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    yes down to a certain temp. in ev, the engine will come on to assist or takeover? i'm not sure how they combine. in hv the same process would start immediately, i would think.

    their first plug in had no heat pump. so if you wanted heat, you would have to run the engine.
     
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  11. winnertakesteve

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    recirculate is a good suggestion, i'll try that. i'd been setting to auto and just trying to keep the temp moderate. the fan doesn't even show as running according to the little ticks on the display, but it definitely is.

    also fair point about a larger car requiring more heat. i do still lament that prius c was taken away as an option in north america, since the prius represents about the smallest thing you can get here anymore. and the cabin if anything feels a bit smaller!

    the extra phev weight is a trade-off for sure, too. in summer all that nice ev range makes it feel worth it. i guess winter is just not this car's forte. mid 20s mpg just feels pretty gross after a decade or so of getting 40+ mpg year round. and we aren't even into the deep freezing temps here yet. its only been in the -5ºC range so far, but jan/feb often can go -20 to -30 for certain cold snaps.
     
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  12. VelvetFoot

    VelvetFoot Active Member

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  13. ChapmanF

    ChapmanF Senior Member

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    Primes have heat pumps.
     
  14. VelvetFoot

    VelvetFoot Active Member

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    As a factor in poor mpg in cool weather?
     
  15. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Recirculate is a good idea if the cabin is already dry and you just want it warmer.

    If you're trying to get rid of moisture (fog on the windows) then fresh air is the better choice.

    A few grams of snow falling off your boots can have a big impact on the cabin humidity... the more you can kick off during entry, the less your car needs to then melt, evaporate, and eventually eject overboard.

    You may be bumping up against the limits of scale. Once the trip is short enough, a smaller car with a smaller engine will always outperform a larger one when it comes to warmup, both in terms of cabin comforts and fuel economy.
     
    #15 Leadfoot J. McCoalroller, Jan 5, 2026 at 8:33 AM
    Last edited: Jan 5, 2026 at 8:44 AM