Hot weather mileage decrease

Discussion in 'Gen 3 Prius Fuel Economy' started by Tom183, Jun 18, 2010.

  1. Tom183

    Tom183 New Member

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    I don't have a lot of data yet, but the last couple of times we had temps over 80F, my mileage took a significant hit - about as bad as when temps are below 20F (more than 10%)... Shutting off the A/C doesn't change things much - any ideas?
     
  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    I am in Mississippi, while my outside air is 95 degrees, my intake tract air is as high as 130 degrees. Warm air is less dense, so less is getting in the cylinders. That is my theory, I actually run the A/C almost continually.
     
  3. mainerinexile

    mainerinexile No longer in exile!

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    Same problem here, also driving in Maine (and NH), as described in a previous post. My mileage has decreased since I bought the car in April, despite warmer weather and now 44/42 psi in the tires to try to improve the mpg.
     

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  4. bedrock8x

    bedrock8x Senior Member

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    Reset the tank average, ie, trip A, then drive without the A/C, you will see the difference. The best mileage is when the outside temp is between 70-80 and have the A/C off.

     
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  5. Tom183

    Tom183 New Member

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    That's what I was thinking as well - I'm just surprised at how drastic it is. My best mpg is around 60-70F max, but it's a fairly gradual drop-off down to 20F. Going up from 70F, the drop-off seems very steep - especially at highway speeds.

    What's you're mileage down there?
     
  6. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    I carry a great deal in my Prius, so I get 47 MPG in fall and spring, 42 MPG in summer and winter.
     
  7. jdcollins5

    jdcollins5 Senior Member

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    It has been in the mid-90's here for the last few weeks. I have not seen but about a 1 mpg drop during this time and my daily 52 mile RT. I attribute most of this drop to the A/C cooling the car down. Once the inside is cooled down and the A/C and the fan slow down to two or three bars on the fan screen, it appears to me that the mileage is right about where it was in the 70-80's a few weeks ago. This is watching the 1 min Consumption screen.

    I was averaging in the high 49's to 50 mpg a few weeks ago and the last tank was at 48 mpg.
     
  8. Felt

    Felt Senior Member

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    Interesting thread. I cannot prove or disprove any theories.

    I live at 5,000 foot elevation. Winter was cold, and my fuel efficiency suffered. When Spring arrived, my efficiency improved significantly. It has not gotten hot yet, so I have no experience in the intense heat, but on a recent 4500 mile trip to the deep south where it was hot, my overall average was 51.3 mpg .... mostly 72-74 mph Interstate travel, A/C operating most of the time. Needless to say, I was delighted.

    I agree that hot air is less dense, but also is 5,000 foot elevation air. I assume the computer adjust for temperature and elevation .... if the air is less dense, wouldn't there be a reduction in the fuel mixture to maintain the fuel/air ratio??

    BTW, I want to "sing the praises" of the A/C in the Prius. It is so comfortable and so quiet .... I really enjoy the fact that the compressor is electric driven, and does not require the engine to be running.

    There are so many negative comments about this remarkable vehicle, I just wanted to make a few positive comments. For example, I love that there is no accessory engine belts to break. I have found the vehicle to be comfortable; quiet; fuel efficient; and no rattles (honest). On the 4500 mile trip I even learned to use the NAV system ..... however, the software leaves something to be desired, but that can be corrected.
     
  9. Tom183

    Tom183 New Member

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    Well, the temps settled back to 70F today, and mpg went back to happiness.

    I'm pretty happy with the car overall and my "lifetime" mileage (since mid-December) is at 45.4mpg, so I'm not doing that bad - but getting to 50 over an entire year is going to be tough if the hot days are around 42...
     
  10. Tom183

    Tom183 New Member

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    So is it only Mainers who are seeing this drastic a decrease in hot weather? I see posts from other (hotter) parts of the country where it doesn't seem as bad...
     
  11. alfon

    alfon Senior Member

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    When I was in Reno last year during our honeymoon the temps were in the 80-s to sometimes low 90's and we still got much better MPG's, over 50 MPG True, than we do in the Rain Belt, North West U.S. , 40-50 degree weather for 9 months of the year. MPG's average about 45-48 true in that climate.

    We even had the AC on most of the time in Reno. I 'll take the warm weather any day for MPG's.

    alfon
     
  12. Tom183

    Tom183 New Member

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    That's what I can't figure out - why my mileage would be WORSE now... Not just a little either - we're talking 15% or so.
     
  13. phoenixgreg

    phoenixgreg Senior member

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    Oh, my mileage has gone down significantly - try running a Prius in 113 degrees. Here's what I have observed: The SOC indicator drops like a rock because extreme heat is bad for ALL types of batteries. We're lucky to get 2 years out of 5 year batteries here (12 volt). So the ICE has to work harder to charge the HV battery for the AC compressor and the 12 volt battery which runs the blower/cooling fans. MPG reductions happen for all cars in this climate during summertime.
     
  14. traveler1600

    traveler1600 New Member

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    I'm bringing my 2010 in wednesday. The mileage has dropped from 49-50 to 40. Same driving, same gas. I live in florida and have been running my AC since January. The batteries always seem to be charging. I used to be able to start, back out my 80 ft drive and get 50 yds down the road before the engine would start. Now I hardly get out of the garage and the engine is running. I can hardly ever get the EV to work. Always says not available. I used to use it in parking lots just for kicks. Never really saved any gas, just drains the batteries and starts the motor sooner to charge. I'll let you know what the deaeler tells me on wednesday. I can already give you my guess. "looks like everything is running normal" "there is a normal variance in Gas mileage....bla bla bla... But I know, the car is not charging or running as effiecient as it was.
     
  15. Tom183

    Tom183 New Member

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    My current hypothesis has to do with the fact that most of our hottest days so far this year have been dry, and I'm pretty sure that dry air carries less heat than humid air. This would mean the ability of the radiator to cool down hybrid components may be less on a dry 80F day vs a humid 90F. (I have also experienced the reverse of this - humid cold air sucks more heat from the ICE than dry cold air.)

    The other thought that comes to mind is that the radiator can only cool components down to (not below) the outside air temp. So even if the radiator is staying ahead, the hybrid components may have increasing losses as the limit temp goes up.
     
  16. Felt

    Felt Senior Member

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    I am sure we can all agree that our mileage decreased in frigid weather.

    The weather here in the mountain west has been high 80's low 90;s and my mileage has been very good ... 53+ mpg. I cannot see the A/C is a major drain except the electric driven compressor obviously required battery power. So too does the radiator fan. I suspect in really serious heat (Vegas, or Phoenix) there would be a drain on the ICE to keep the fans and compressor operating.

    Let me once again praise the A/C in the Prius. Best comfort we have ever enjoyed.
     
  17. LisaLisa

    LisaLisa Junior Member

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    I'm in Austin, TX. Starting in May the temps get up into the 90's and with the humidity, we are now in mid July with heat indexes of 110. And my mileage has plummeted to around 37-38 mpg. Ordered new cabin filters, but that's just for maintenance. Tires are 40/42 and I'm wondering if 42/44 would make any difference? The car would be a tish higher and maybe get a little more air?

    Lisa
     
  18. janice45

    janice45 Dudley Girl

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    My mileage has gone done also but I'm holding at 50 .It took a dive the last week in June right after my oil change .Before the oil change I was getting 57 and better.
    I know i should not complain but i was waiting to see 60 not 50 and under
    This is not my first prius and never ran into an issue of weather before .
    I checked with the dealer and heard some non interesting reasons why there was a drop in mileage.I give up .
    Can the hot weather really make this much difference?
     
  19. macman408

    macman408 Electron Guidance Counselor

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    As has been noted before, overfilling the oil can have a significant negative impact on your mileage. I suggest checking the dipstick to make sure it's no higher than the high mark. Some people fill their cars only to half or 3/4 between the high and low marks to get better mileage.

    If it's too high, take it back and get it fixed by whoever changed it.
     
  20. Bob64

    Bob64 Sapphire of the Blue Sky

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    Here's another plausible explanation as to why too hot isn't good for mpg: 1) Inverter not at peak efficiency due to high temp 2) Battery efficiency (available capacity/ability to recharge/ability to supply power) reduced due to high temp
     
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