Hot Weather MPG hit! OUCH!

Discussion in 'Gen 2 Prius Main Forum' started by Fubar1764, Jul 5, 2013.

  1. Fubar1764

    Fubar1764 Member

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    Here in Houston, Texas (it's bigger than France), we only have two seasons: summer's coming and summer's here. In the past month, we had a mini-heat wave which took temps up to 100+ for about two weeks. My indicated mpg dropped from about 44 to 39. Whoa. That's over a 10% drop. Got me worried something was wrong. In the last week, the temperature has dropped down to the 90s, and the mpg is now back to the low 40s. In both temps, I leave both rear windows cracked open when I'm parked to dissipate the cabin heat in the sun. Got me thinking, "What was the difference?" Wasn't the driving, wasn't the air pressures (which should go up as temps get higher). The main difference was a/c use. When temps were in the 100s, I was blasting the a/c at LO and highest fan speed. In the 90s, I have it on LO and at medium or below.
    Moral of story: heat will destroy your mpg if you want to be comfy!
     
  2. uart

    uart Senior Member

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    You got it right there Fubar. I find that light or moderate a/c usage doesn't effect the fuel economy too much, but when the heat gets bad enough to need "full blast" then it can really take a toll on the ol' mpgs.

    Around here we get a lot of high humidity heat, so often just the lowering of cabin humidity that the a/c provides can keep things reasonably comfortable without setting it too low. When it's not too bad I set the temp pretty high, about 26C (79F) and the fan low/medium. With these settings the hit on MPG is often barely even noticeable. But I know what you mean, sometimes the heat get so bad that you've just got to go full blast.
     
  3. Skylis A

    Skylis A Senior Member

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    Does yours have leather seats? If it gets that hot and 100+ gosh I'd need the air conditioning to pre-cool the car before I sit down :eek: or I'd use a towel
     
  4. richard schumacher

    richard schumacher shortbus driver

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    Gotta do it though. The traction battery system gets its cooling air from the cabin, so if you're too hot so is the battery. Take comfort in knowing that, just as in very cold temps, conventional cars are doing worse.
     
  5. MPGnutcase

    MPGnutcase Active Member

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    I run the A/C with the ECO button on which does not seem to hurt my MPG
     
  6. N3FOL

    N3FOL Member

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    Yep. AC would be running on this end too, but we try to drive as smooth as possible to maximize mpg. Comfort is more important to me on a hot summer day than mpg. Drive Safe.
     
  7. Fubar1764

    Fubar1764 Member

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    In the last 9mo, I've put on 30k miles on her. This is the first time I've consistently dipped below 40mpg.

    Yes, I do have leather. She's a package 5, just one step below touring. I'll keep updating this thread as the summer rolls on.
     
  8. N3FOL

    N3FOL Member

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    After 30K, try to replace your air filter and cabin filter. Drive as smooth as possible.

    My TCH remains steady at 42 mpg during this hot season.
     
  9. Skylis A

    Skylis A Senior Member

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    Low 40s with temps in the 90s sounds about right. I did some shopping today with 91F up here. Depending on the shopping trip, I got ~40 mpg, because each trip was short and I needed to cool the car off with the air conditioner each time. I had ECO on, but, I didn't want my sweat to soak into the cloth seats in my car, so I used a modest bit of air conditioning. (For the curious: the cloth seats in mine were very warm, but never scorching hot, and they're still clean.)
    Yeah, big MPG hit.
    Oh well, that's still better than the 12 mpg I got with my old SUV and with the engine still on at all the red traffic lights.
    We get 1 month of summer here anyway.
     
  10. Fubar1764

    Fubar1764 Member

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    We had a short respite from the heat. Highs only 100 and lows of 78 at night. During the day, it's typically only 93. MPG of last tank was 42.8. Not bad!
     
  11. MPGnutcase

    MPGnutcase Active Member

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    Check your A/C and cabin filter I run mine on ECO and it cools the car very nice
     
  12. Fubar1764

    Fubar1764 Member

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    The G2 Prius doesn't have an ECO option. Cabin filter was changed about 30k ago, so you're right, it's due. Hadn't thought about that. Thanks!
     
  13. MPGnutcase

    MPGnutcase Active Member

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    your welcome :cool:
     
  14. dorunron

    dorunron Senior Member

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    On that 107 degree day that you referenced I was out all day in it. The lowest I had to run the A/C was on 72 with a fan speed of M and the recirc on. Kept it real cool, but I also keep my cabin air filter clean. It only takes a couple of minutes to pull it out and knock the leaves, dust etc. out of it. If you haven't done yours in 30K, it's no wonder you had to run your A/C on full blast at LO that day.

    Maybe it's overkill, but I check and clean that filter every two weeks when I check the tires. I keep the tire gage in the glove box, and it only takes a couple of minutes time to service that filter. A clean filter will let the A/C work better which will keep you and your traction battery a lot cooler with less effort and less electricity which ultimately saves gas too.

    Ron
     
  15. edthefox5

    edthefox5 Senior Member

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    30,000 miles in 9 months is super duty use. You never mentioned the total mileage but your due for new transmission fluid and way overdue for Inverter Coolant change.

    Fresh Inverter coolant will get your mpg's back. The cooler the Inverter the better gas mileage you get.
    The Inverter is the heart of the Prius. And changing the I Coolant is very very easy.

    Make sure you use or buy the front window heatshield at the PC Shop here. Its the best, custom fit to the Prius, insulated, and is still going strong after 6 years of use. It will keep the car interior very cool.

    Lately I have not been able to store my cars in the garage due to home improvements so was concerned the Hybrid battery was getting very hot sitting in the sun so cut up a aluminum foil covered insulation board (they come in 4x8 sheets at HD) to sit on top of the pull out luggage screen covering the trunk. Like 38 X 25. That really helped cut the heat down coming in from the rear window and making that trunk area hot. The front part of the board is raised so the view of that board in the rear view mirror is oblique to the driver. I leave it installed when driving also. Works great.

    Car has very high quality tint job too but the tint, window shade and this board really helps if forced to park in direct sunlight. Takes alot of energy to cool down a heat soaked interior. That and my car always has fresh conditioned Inverter Coolant on board.

    Oh and since yours is an 08 check the condenser face for bugs and fod and dirt. Take top plastic engine cover off and buy blue can of spray coil cleaner at HD. Hose the condenser down, then hose it down with cleaner, let it sit and then rinse off.
    AC will be about 20% colder. Same goes for house only the house gets about 30% colder if done right. Do both of mine every summer. Really pays off.
     
  16. Patrick Wong

    Patrick Wong DIY Enthusiast

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    If you have the temperature control set to MAX COLD then the AC compressor will be operating constantly. That obviously will result in a big mpg hit.

    I am wondering whether the refrigerant level in your air conditioner might be low, which causes the system to work much harder to achieve a given cabin temperature.

    I also am dealing with 100+ degree temps in the summer, here in southern AZ. Today for example the Prius thermometer read 106 degrees around 3:30 PM in Tucson. By the time I got home at around 6 PM the temp was 100 degrees. I'm getting mpg in the mid-40s during the summer, and high 40s to low 50s in the fall and spring when I don't use the AC and ambient temps are moderate.

    I personally find that if I leave the AC AUTO function on, which automatically controls fan speed, and then set the temperature control to 76 degrees, this results in a very comfortable cabin environment. YMMV of course.
     
  17. MPGnutcase

    MPGnutcase Active Member

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    106 wow that's a hot one :LOL:
     
  18. Fubar1764

    Fubar1764 Member

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    When I got my Prius 9mo ago, I did all fluid changes. Tranny, inverter, oil, the whole smash. I figure it's cheap insurance against _______ failure. Kingdom failing for want of a nail and all.
    I'll have to try hosing down the condenser as suggested. Yes, I'm overdue for a cabin filter change as well as a tranny fluid change as well. Inverter change? Interesting. I guess every 30k on that too.
    When my mechanic comes back into town, I'll be doing all of this and will report back. For what it's worth, the break in the temp in the last few days has me back up to mid 40s on the computer.
     
  19. Patrick Wong

    Patrick Wong DIY Enthusiast

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    I would wait until you hit 100K miles on the odometer for the first change; then change the coolant at 50K mile intervals after that. If you have evidence that the inverter coolant pump was already replaced under the recall, then the coolant may have been replaced already. For example, my repair receipt shows that Toyota was charged for a gallon of SLLC, but that does not guarantee that any new coolant actually was added...

    Ed previously offered a reasonable suggestion to put used coolant into a clear container to gauge whether it contains rust, other sediment or whatnot - and thus provide another means to decide whether to change the coolant. I had found that when I earlier changed the coolant on my 2004 at 100K miles, it looked like something you would be willing to drink if poured into a punch bowl. YMMV.
     
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