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Significant drop in mpg with AC

Discussion in 'Gen III 2010+ Prius Fuel Economy' started by shankyyy, Jul 9, 2010.

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  1. shankyyy

    shankyyy Member

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    I went on a 1200 mile+ OH-DC-NY-OH road trip the last weekend, and while my onward journey was showing 48+ mpg on the HSI with AC at mid levels (not Auto), during my return drive in 100F temperature, for a 120 mile stretch after filling gas, the HSI showed only ~35mpg?? Is this a normal thing, i.e. mpg dropping by more than 10mpg with AC? I never saw this much drop in my corolla.
  2. vday

    vday Member

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    Was there an altlitude change?
    Hills kill MPG
  3. JimboK

    JimboK One owner, low mileage

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    That much of a drop in fuel economy with AC in these temperatures can be expected in city driving but not on the highway. My first guess is that you had a tailwind going and a headwind coming back. Even a mild wind can have a significant effect at highway speeds. I also would consider whether net elevation difference is a factor.
  4. mikepaul

    mikepaul Senior Member

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    I have a now-ancient 2004, but I think the issue will be the same: REALLY hot weather will keep the A/C cranking far longer than just 'warm' weather will, and the engine will run a whole bunch more to maintain the battery level. Usually you can cruise down a higway at 70MPH on battery-only off-and-on to keep the MPGs up, but when the battery wants juice that 70MPH on gas isn't quite economical anymore...
  5. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    I average a 5 MPG drop here in MS in the summer, 47 to 42 MPG, but have not dipped into the 30's as an average.
  6. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator Staff Member

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    Yeah, I see a drop in MPG with the A/C but it's only an issue when I'm commuting on streets, not when I'm at highway speeds. I agree with Jimbo. There's something else going on there. The headwind/tailwind idea might have some merit. I experienced that in a big way during a long-distance drive and back last year.
  7. jdcollins5

    jdcollins5 Senior Member

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    I agree with the others here. I am only seeing about a 3-4 mpg drop and the weather has been in the high 90's all week. The biggest hit that I get on the daily commute is in the afternoon when the A/C has to cool the cabin down after sitting in an open paved parking lot all day. Once the cabin is cooled down and running at 60 mph highway speeds, the mpg will actually start picking back up again. My mpg has only dropped from an average of 50 to about 46 during this time.
  8. tumbleweed

    tumbleweed Senior Member

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    As others have stated I have never seen anything even close to a 10 MPG drop because of AC use on the highway. I do most of my driving on the highway and frankly I usually don't see much difference due solely to AC, but I set the temperature at 77 to 80 and drive in the ECO mode.

    What temperature do you usually select when running AC? As mentioned above headwind is a mileage killer. Did you check your tires maybe one is low? You could have gotten some gas with more ethanol than usual, that could account for some of the drop.
  9. dbcassidy

    dbcassidy Toyota Hybrid Nation, 5 Million Strong

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    Headwinds, tire pressures, a/c running and what type of gas (ethanol added) will drop your mileage. When I fill up in MAss - 45/47 mpg. When I fill up in New Hampshire - 52/55 mpg. The only difference i can see is New Hampshire gas has less ethanol in their mix.

    My driving is 50/50 highway and stop and go.

    Dbcassidy
  10. shankyyy

    shankyyy Member

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    I was driving thru' I-78/I-76. The I-76 thru PA is that killed the mileage. I had kept the temperature to 72; then seeing the low mileage, i increased to 75. still no effect. I-76 was a moderately hilly region but not anywhere close to the onward drive through the appalachian range in WV. Anyway, eventually when i reached columbus,OH the HSI showed about 47 mpg. My first thought was a combination of heat + maybe poor quality of gas. I filled at Citgo in Harrisburg,PA.
    Couple of interesting points
    - During my onward journey thru' Appalachia, where I clearly lost power (see my other thread in the main forum), I still averaged about 49-50mpg on the HSI. Of course,the ac was only about at mid-level, and OATs was in the 80s/low 90s.
    - Today i had my first oil change, filled gas at Shell, drove for about 15 miles stop-n-go with a/c kept at mid-level, and the mpg is only about 35mpg on the HSI. I used to get 48-50 on the HSI with the a/c on, and more than 50 with a/c off but 30s are really concerning. I live in plains, and today the temp was only in the 80s; so I cannot explain how mpg could be this low.
  11. SageBrush

    SageBrush Senior Member

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    The list is long.
    Start with manual brake and tyre pressures.
  12. shankyyy

    shankyyy Member

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    Manual brake-could you explain what you mean here?
    I keep tire pressures at factory recommended. And before my road trip last weekend, i made sure it was ok.
  13. cwerdna

    cwerdna Senior Member

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    The other figures you cite don't sound bad, as for the above, the problem is your 15 minutes of stop and go. Both factors will kill your mileage along w/the extra drain on the HV battery caused by the AC compressor.

    I can't speak to the compressor draw on the 3rd gen, but on my 2nd gen, idling w/ICE off pulls 1.3-2 amps from the HV battery (at ~220 volts). The AC compressor can make it go as high as 9.x amps.

    Since you mention oil change, make sure they didn't overfill and used the right viscosity.
  14. tumbleweed

    tumbleweed Senior Member

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    This thread may be relevant. http://priuschat.com/forums/gen-iii-2010-prius-fuel-economy/68603-same-prius-not-same-mpg.html Some of the Gen3 cars may get significantly worse mileage than average and we don't know why, at least not yet. If this continues it could indicate a problem starting to develop, or maybe it's just something simple that hasn't been checked yet.

    Several possible reasons are proposed, none tested yet, i.e. running in open loop, valve in exhaust system heat exchanger stuck shut, etc. If the problem continues it might be a good idea to have the codes read.

    At the risk of being controversial I might also mention that most of us on this forum have found it improves mileage and handling when we increase the tire pressure above what is recommended by the factory, keeping it under the max load pressure listed on the sidewall. There are plenty of threads on that subject and I only mention it in case you haven't seen them yet.
  15. shankyyy

    shankyyy Member

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    I checked the oil, and for the first time ever since I owned any car, i could not say for sure what the actual oil level is. On one side of the stick, upto the full mark the oil covered the entire width of the stick, and for an inch or so beyond the full level, there was some oil but not covering the entire width. On the other side of the stick, the oil was less than the full mark but not covering the entire width. I tried all ways to get an accurate reading but right now, but nothing worked. I am not sure whether the oil is more or less now! The service sheet says 5 quarts, and our manual says only 4.4 quarts is required.
    Tire pressure is 37 on the front and 35 rear.
  16. tumbleweed

    tumbleweed Senior Member

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    I have found the best way to check the oil is to park the car where it is level for several hours or overnight, then pull the dipstick out and take the reading with out wiping it off and putting it back in again. When you first shut it off and try to check it oil gets all over the top of the dip stick just as you describe.

    It takes 4.4 quarts, I put in about 4.3 which leaves it 1/4 inch or so below the full mark.

    EDIT: If you find it is overfull I recommend going back to the dealer and having the extra oil removed, it will effect the mileage and could even cause engine problems.
  17. shankyyy

    shankyyy Member

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    i checked the oil this morning after it was being parked all night but now that you say, my parking lot is not that flat. i'll check again tomorrow at work, where the parking lot is more on a level surface. i'll probably drive for a couple of more days, and if the mpg does not improve, take it to the dealer.
  18. cproaudio

    cproaudio Speedlock Overrider

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    high AC setting kills MPG. I just got my 10 Prius a few days ago. Yesterday the temp was around 103F. I had my mom with me and I didnt want her to sweat so I turn up the AC for her comfort. I had the AC at 3 sometimes 4 bars setting. My I ended with 47MPG. I was pissed. I was hypermiling too. Today the temperature dropped to 99F. I was by myself. I could stand a little heat so I had the AC on the low setting. My all day trip averaged out to 62MPG.
  19. Tom183

    Tom183 New Member

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    I experienced EXACTLY what you describe a couple of months ago (huge/20% mpg loss on a highway trip on a hot day), and it wasn't due to tires or any of that other crap, because none of that had changed. However, it was the first time I had to rely heavily on the A/C (had used it a little before then, but not full-blast and not for very long).

    So my WAG would be that there's a break-in period required for the A/C, but I think it's at least equally likely that "TheSpoils" could be onto something - the battery (or more likely, the inverters) may be getting too hot and doesn't get enough cool air from the cabin (and that would be even worse if you were trying to limit the AC use or turned it off). It's a lot easier to cool down the cabin air than the rows & rows of Li-ion & plastic blocks.

    I haven't had that problem again other than that one time (hence the "break-in period" hypothesis), but I haven't done a similar drive in similar conditions since then either - been doing lower-speed, shorter-distance drives in somewhat lower temps. And mileage has been great - I'm working on a 60mpg (displayed) tank right now after 110 miles (and that's with some moderate A/C use).

    But if that problem seems to be happening again, I'll take the suggestion of keeping A/C on and disabling recirculation so that the battery gets more cool air.
  20. socratesthecabdriver

    socratesthecabdriver Member

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    i get a 2 liter to 100km increase runing the ac on the first 2 klics and on the low temp setting.i call the ac the big bear.it eats everything in its way thank god it hibrenates in the winter!
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