How far off is my fuel economy?

Discussion in 'Gen 3 Prius Fuel Economy' started by TheChip, Jun 24, 2018.

  1. TheChip

    TheChip Senior Member

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    I'm just wondering how much lower this is compared to most out there.

    High AC the whole time, 18 inch wheels and Continental Extreme Contact Sport tires. Ambient temperature is 92 degrees, all highway with 3 stops along the route. Filled up with Shell e10 right before I hit the trip reset.

    I feel like my MPG took a hit after installing my OCC and cleaning the manifold and EGR pipe. IMG_20180624_152224704.jpg
     
  2. jdcollins5

    jdcollins5 Senior Member

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    Depending on the highway speeds, I normally average ~42 mpg at 75-80, ~45 mpg at 65-70 and ~50 mpg at 55-60 with a stock 2010 Prius III.

    What size tires came on your Prius II? My guess is 15 inch. How much is your speedometer off due to oversize tires?

    What mpg were you getting before the OCC install?
     
  3. TheChip

    TheChip Senior Member

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    speed is 100% accurate. 75 mph speedo just measured at 75 GPS.

    The stock wheels were indeed 15.

    Before the EGR pipe cleaning I was getting 42 highway 65/70 so I've been sitting at 35 and scratching my head at how that could have made any difference.

    We tried to get the EGR itself off but couldn't break the bolts in the rear. I can't imagine that would cause this.
     
  4. Grit

    Grit Senior Member

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    Answer, what is larger diameter tire.
     
  5. jdcollins5

    jdcollins5 Senior Member

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    The stock Prius speedometer is typically 2 mph optimistic for safety reasons. At 60 mph speedometer the actual speed is ~58 mph. Your oversized tires must have increased the mph by about 2 mph to match the GPS speed.

    Hopefully some of the members that have installed the OCC can help you with your loss of mpg.
     
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  6. fabio

    fabio New Member

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    That is correct and I can attest to this. This is actually true for ALL cars not just the Prius. So when you get a speeding ticket and you know the speed you were going. Subtract -2 MPH if the officer thinks he knows your speed. By default the KA speed detectors will subtract 2 from your speeding but some officers know this and they'll add 2mph. :)

    Go back to the stock tires.
     
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  7. bisco

    bisco cookie crumbler

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    18 inch tyres.
     
  8. TheChip

    TheChip Senior Member

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    So I'm gunna go ahead and post the solution to my issue, in case anyone comes across this thread. Bit embarrassed but if it helps someone else, so be it.

    If it's in the upper 90's and you're wondering why your fuel economy is taking a nose dive, consider not running the AC on the "low" setting with the fans on full blast.

    Apparently that causes a fairly major hit in fuel economy. I was always running on max AC without a second thought. Now my mileage is back near where it's supposed to by keeping the AC on auto at 75-77 degrees, and completely normal with the AC off.

    I guess I figured since it was an electric AC compressor it wouldn't have mattered, but it does seem the current draw of the compressor causes the battery to need a lot of recharging while driving, leading to poor mileage. So if it's hot as hell and you're wondering why your mileage is down, take a look at your aircon habits!
     
    #8 TheChip, Jul 19, 2018
    Last edited: Jul 19, 2018
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  9. bisco

    bisco cookie crumbler

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    keep in mind that the hvac system helps keep the battery in good health, and prius batteries in the south suffer from heat exhaustion.
     
  10. Grit

    Grit Senior Member

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    Anything that draws power high amperage 12v will significantly reduce MPGs, the main reason why I dont like driving with low beams on, will only turn them on when its really really dark. No LED or HID low beams on my prius yet, so still 55 watt per low beam bulb. Dont want to upgrade to LEDs or HIDs for looks but would do it for 35 watt per low beam side to avoid ICE to begin it's full HV charging cycle. 1st world problems.
     
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  11. Threej

    Threej Member

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    This doesn't make sense to me-- keeping the AC on auto at 75-77 degrees in the upper 90's result in it running the AC on low with the fans on full blast, the very thing you say to avoid.
     
  12. bisco

    bisco cookie crumbler

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    'low' is the lowest setting on the temp display. setting it to low results in max a/c. setting it to 75 runs the a/c more moderately to bring the temp in the car to 75 and uses less battery power.
     
  13. Threej

    Threej Member

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    Ahh! Sorry, I confused setting the temp to "Low" with running the compressor on low, which the AC attempts to do on auto when in eco mode.
     
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  14. Lucifer

    Lucifer Senior Member

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    15” 195/65 tires are the correct size for prii, and the speedo reads correctly.
    If the op has a correct speedo reading, his tires may be wide, but not oversized in circumfrence, which is what matters.

    A 2010 getting 30’s mileage....time for a new traction battery, not a rebuild.

    And reclean the manifold, occ? And egr, new plugs, a Toyota obd2 read, trickle charge the 12v overnight, replace the 12v if it’s origional, cycle the emergence brake a few times and clean the cabin air filter;)

    After a new traction battery even with the 18’s the mpg’s will be touching if not in the 50’s.
     
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