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Why the drop in fuel efficiency?

Discussion in 'Gen II Prius Fuel Economy' started by cashmerewhore, Jul 26, 2011.

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  1. cashmerewhore

    cashmerewhore New Member

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    I've noticed that since my last oil change, my 2008 Prius is averaging (according to it's own computer, not my calculations) 39-41 MPG by the time it's time to refill the tank. I purchased this car last summer, and while I've been hard pressed to get the EPA records, usually in the summer with the A/C running I could average 45-48 MPG.

    This reduction upsets me. I have recently moved, so I've changed mechanics and I've noticed that they completely filled the oil after the change (almost a month since the oil change it hits right at the full dot). I may also be (unknowingly) using an ethanol added fuel -- just saw today that it can affect fuel efficiency. Tires are from Firestone, they have approximately 20K miles on them (they were replaced when I purchased the car).

    Any other suggestions as to why I'm getting winter fuel efficiency mid-summer would be greatly appreciated. I don't expect to break 50, I tend to drive my Prius a bit faster than most.... and living in Ohio with allergies limits how often I drive with the windows down. I do usually set my climate control to 70 degrees, fresh air and medium on airflow. Currently the car is not parked in a garage. I'd love to get my 5MPG back!
  2. bisco

    bisco cookie crumbler

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    you may have nailed it. ethanol = 10% reduction. and your new route may not be helping. go thru the 'why am i not getting expected mileage' questionaire for more suggestions and welcome to the club!
  3. striker308

    striker308 Prius owner

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    Not sure if this helps or not, When NY changed the additive in the fuel mix. I dropped more than 3 MPG. when using the 10% Cornflake mix I get even worse.
  4. F8L

    F8L Protecting Habitat & AG Lands

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  5. Danny Hamilton

    Danny Hamilton Active Member

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    It sounds like you've already given your most likely reasons in your explanation.



    I assume that this move means a change in typical weekly driving patterns? Perhaps more shorter trips and less longer trips than before the move? Perhaps a bit more stoplights, stop signs, and stop-and-go traffic than before? Whatever it is, I doubt that your driving patterns are exactly identical to what they were before the move.



    I believe Ohio has had a very hot July. Your A/C has probably had to work harder than usual to maintain the interior temp at 70.



    Does this mean that you used to keep it in a garage? Perhaps being parked in the sun more often now than before means that the interior starts out hotter on average than when it was garage kept? This would mean that initially the A/C would have to work harder just to cool the car down in the first place.



    Yes, if the gas stations you typically used before the move had no added ethanol and the gas stations you typically use now do have ethanol added, it will have an effect on MPG.
  6. cashmerewhore

    cashmerewhore New Member

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    Yeah, Ohio has been pretty brutal in the past few weeks as far as climate goes. Even the showers haven't helped with the temps being in the 90s throughout the day (upper 80s after the sun goes down).

    What fuel economy are you getting and how are you determining fuel economy? --trip computer

    - What fuel economy are you expecting and why? -- 45-48MPG, what I was receiving last summer

    - What are the approximate outside air temps? Days: 90s, evenings/mornings, upper 80s.

    - How long are your trips? 15-20 miles, mixture of city & highway

    - How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving? 20% city, 80% highway for work. Not much stop & go traffic.

    - What region/state are you in? (if you haven't set your location in your profile)

    - What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.) -- Boring & flat.

    - Is your oil overfilled? (i.e. above the full mark on the dipstick) -- right to the full dot.

    How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night? --Battery was replaced in December 2010.

    Have you had your alignment checked? Any pulling or abnormal tire wear? -- No.

    - Are you using the factory tires and wheels? If not, please indicate tire make, model and size (e.g. Goodyear Assurance Fuel Max 185/65R15). Firestone, it's seriously too hot to go look at them right now.

    - What are your tire pressures? -- I should know this....

    - Make, model, year, engine and transmission of previous car? (e.g. 08 Honda Civic Si 2.0L 4 cylinder, manual transmission) What did you actually get on the same trips/commute? (Please give us actual numbers, not EPA ratings.) 2004 Chevy cavalier LS sport (4 cyl, automatic). Can't judge on same commute since I moved, but when I lived at my previous residence I was getting no more than 24 MPG in the cavalier, same commute in the prius would land the 45-48 MPG.

    - How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking? If I'm driving through neighborhoods I try to stay electric only. I need to coast better, I do brake rather late.

    - Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on? No.

    - Are you driving using D or B mode? -- D

    - HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to? AC set to 70.

    - If reporting a mileage drop, did anything significant change on your car (e.g. accident, hit a curb or big pothole throwing off alignment, oil change/other maintenance/repairs, changed tires or wheels, etc.) or your commute? I did bubble a tire this winter on a pothole, so I should probably get the alignment checked...
  7. richard schumacher

    richard schumacher shortbus driver

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    The first thing to do (as always) is to check the tire pressures. The new mechanic is probably setting them to 35 PSI. They can safely be set as high as the value given on the sidewall of each tire, although the ride and handling will change.

    The ethanol in E10 fuel cannot reduce MPGs by more than 3%, except possibly for the first few fillups during which water that was in the tank is being removed by the ethanol. So that is not the explanation.

    I easily get 45+ MPG with drivetime temps running 90'F to 105'F and the AC set at 75'F .
  8. cashmerewhore

    cashmerewhore New Member

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    I changed the air filter with my fill up today and made sure to use gas that was not labeled "contains up to 10% ethanol". With the remainder of the driving I've done today I'm averaging 45 MPG with the air still running (thank you Ohio heat wave). So, I'm glad that it seems I've got my old average back.
  9. Capt Ed

    Capt Ed Junior Member

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    Hi Richard! I must disagree with your statement. Almost everyone here in FL. I talk to says they lost 10-15% with the E10 we are required to buy. I personally experienced that with my Lincoln Navigator and Lincoln Town Car. ( traded in the Town Car for my 2010 Prius III).
    Ed
  10. F8L

    F8L Protecting Habitat & AG Lands

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    They must be doing something to the fuel in your area because no one else around my area or even on this forum experiences such a drop in MPG. How are you testing this idea?
  11. uart

    uart Senior Member

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    Do you have any update for us on what tires you've got. As in the exact model and size?
  12. usbseawolf2000

    usbseawolf2000 HSD PhD

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    I think it is a combination of AC set to 70 deg F with the mixture (including heavy congestion?) driving. The tire pressure may also play a role so make sure you check the PSI.

    Here is a tip if you drive alone. Once the cabin temp is lowered (from initial inferno), close the two AC vents on the passenger side. That will double the air flow on the driver's side. Point them directly (as per your comfort) to you. I find AC set at 78 deg F comfortable this way. I prefer 76 deg F but on a really hot days above 90, 78 deg F is very cool.
  13. uart

    uart Senior Member

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    I would claim that the simply not knowing what type of tires you have statistically costs you about 2 to 3 MPG. I say "statistically" because there's a strong correlation between not knowing what tires you have and having non-LRR tires.
  14. nerfer

    nerfer A young senior member

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    I agree. 70'F seems a bit much, the A/C must be running full blast the whole time.

    I keep mine at 76-78'F when it's too hot to roll down the windows (>86'F) or driving at high speeds. I don't like hot temps at all, but 76 is good enough (I'm an efficiency junkie).
  15. wick1ert

    wick1ert Senior Member

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    I just got done using my brothers Jeep for the past month, not running ANY A/C. Now that I've got my car back, even 78 was chilly yesterday and I bumped it up to 80. I find that I tend to start higher, and if I need to adjust during a trip will do single degree increments until I feel more comfortable.

    I'd start with a higher initial temp setting and see how that affects things and if you are still comfortable at a higher setting.
  16. usbseawolf2000

    usbseawolf2000 HSD PhD

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    I am posting this in case the OP is not aware. Plus it is important to mention it.

    For a regular car (like Jeep), there is no fuel economy incentive to adjust the AC temp. The AC compressor runs off the gas engine with a belt. The faster the gas engine revs, the colder the AC gets. If you want warmer air, no problem - just mix it with some hot outside air.

    Prius AC is full electric with compressor that can run at various speed according to the temp demanded. So there is incentive to conserve and be creative.
  17. LIPriusFreak

    LIPriusFreak Can I haz JDM?

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    i get 41.3 mixed mpg (as per MFD) in my 06 prius with 87k...I have brand new air filter, use Mobil 1 5w30, warm up almost all the time, drive it like i stole it (usually), i run nothing but Shell 89 V-Power gas, my a/c is cranked between LO (64) and 71 degrees, and I run oversized 195/60/15 LRR Goodyear Assurance fuel max tires pumped up to 40 psi all around...

    EDIT: oh and my car has had all recalls/scheduled service performed (30k/60k and 90k was done early b4 i bought car)

    I attribute my gas mileage to my fast driving habits combined with my short work commute (8 miles each way)...I could complain about 40mpg in the prius, but if I got that in my Legend let's say, I'd probably kiss the next random person walking in front of me. So I'm gonna say I'm ok with it
  18. cobaka

    cobaka Junior Member

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    My Prius gets better MPG when on the recycle inside air (not the fresh air setting). I get about 48-51 depending on city vs. freeway.
  19. Capt Ed

    Capt Ed Junior Member

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    Hi! No testing, just speaking from experience and talking to others that track their mpg when they fill up. I have also read replies to readers from the automotive guys in the newspapers that agree with the 10% loss. Florida is considering E15 and scaring the heck out of us. Aside from the mileage loss, the E10 is tearing up all the rubber fuel system parts in our garden power equipment. Bad stuff, E-gas, and our Government subsidizes it! :mad:

    Ed
  20. richard schumacher

    richard schumacher shortbus driver

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    They are confusing it with other causes; it's easy to do. E10 [1] contains 97% of the energy of gasoline by volume, and is a fine pollution-reducing oxygenator and injector cleaner. (Note that this does not explain or excuse our idiotic tax policies regarding fuel ethanol used to make E15 and up.)

    [1] thanks to corwyn for pointing out my previous error
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