Why Don't I Get The EPA Mileage – 2006 Prius

Discussion in 'Gen 2 Prius Fuel Economy' started by Nature Boy, Feb 17, 2025 at 1:11 AM.

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  1. Nature Boy

    Nature Boy Junior Member

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    Ok good folks, so.. I’ve read through the “Why_Don't_I_Get_The_EPA_Mileage” article and checked all the boxes (conservative driver, minimal A/C or heater use, new Costco tires, etc..), but still I’m getting only 36.8 mpg! (and have since 89,200 miles)

    On a ~2500 mile trip I made this time last year, with a car-top carrier and car full of stuff, I averaged 41.3 mpg (I kept track, kept gas receipts etc.), and was getting this pretty much since I purchased the car from first owner (1.7 years ago) who took meticulous care of the car—spreadsheet and receipts recording all repairs (often unnecessary, preemptive). I purchased JJ (short for Jeremiah Johnson) at ~85,000 miles. Note: Previous owner did not drive the car much for one year prior to sale as he bought another hybrid that he drove mainly.

    Anyway, when I got back from my trip, I “went to town” and did the following at 89,200 miles:
    • Replaced plugs with these: NGK # 4996 -- Laser Iridium Spark Plugs -- IFR5T11
    • Replaced downstream O2 sensor with this: Denso Oxygen Sensor 234-4623
    • Replaced upstream O2 sensor with this: Marsflex Air Fuel Ratio O2 Oxygen Sensor Upstream 234-9056 Compatible with Toyota
    • Replaced engine air filter with this: EPAuto CP846 (CF9846A) Replacement for Subaru/Toyota Premium Cabin Air Filter
    • Cleaned out the MAF sensor with this: CRC Mass Air Flow Sensor Cleaner
    • Took out and meticulously cleaned the main battery fan (wasn’t that dirty actually).
    • Replaced PCV valve with this: Beck/Arnley 045-0329 Pcv Valve
    I did all this stuff because the previous owner had not, and it has been suggested as regular maintenance at that mileage and to improve mpg. Of course I also replaced the oil and filter and have done it again since, now at 93,500 miles. Oh and I replaced the tires (Costco, Goodyear, kept at 36psi all around) and got an alignment (purchased lifetime from Firestone; aligned once again since).

    In addition:
    • Auxiliary battery fairly new and in good shape.
    • Confirmed that brakes are not sticking.
    • Fuel treatment provided at 89k
    • Fuel injection service at 77k
    • Transmission drain and fill at 77k
    Now when I purchased the vehicle I purchased the VeePeak bluetooth OBD tool with Dr. Prius. Dr. Prius told me at purchase that I had ~60% life left in the battery (think it said “good” condition). Fast forward to today (~93,500 miles) I’m getting a read of 56% (“fair” condition).

    I’m in Sacramento CA where temperatures are certainly warm in the summer and mild to California-cold in winter. I store the car in my driveway on the east side. I run the car on average 10 miles a day. Car idles/runs smoothly with decent power (for what it is, haha) and with no engine or battery codes.

    So why am I not getting at least 45 mpg? Simply the aging battery? I tend to think that in my big maintenance job, I missed a step or something went awry that I’m not seeing to explain the drop in mpg, but I don’t know. It’s just embarrassing to have a well-maintained and good-running 2nd gen getting such low mpg! :) Thanks for you input.

    Ps. See attached today’s Dr. Prius screenshots and a pic of JJ.
     

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    #1 Nature Boy, Feb 17, 2025 at 1:11 AM
  2. bisco

    bisco cookie crumbler

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    Welcome!
    I would start by replacing all the aftermarket parts you installed with genuine Toyota from a dealer.
    Low hanging fruit first, yes the battery is contributing.
    Check/replace air and cabin filters.
    Check all wheels for drag.
    Get the 12v load tested.
    Make sure the battery cooling inlet is clean and check/clean the intake fan.
    Check the tyre model to see if they are low rolling resistance.
    Consider a grid charger to revitalize the hybrid battery.
    All the best!
     
    #2 bisco, Feb 17, 2025 at 2:37 PM
  3. BiomedO1

    BiomedO1 Senior Member

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    tires, upstream O2 sensor, short hops and fuel quality. but it couldn't be that the car is going to be 20 years old and not in it's prime? Your mileage is around what others are getting, see link below. How you drive also makes a big difference in your mpg. The wrong O2 sensor will effect mixture, probably too rich. You'll need a much better scan tool to look at the fuel trims.
    2006 Toyota Prius MPG - Actual MPG from 402 2006 Toyota Prius owners

    A new traction battery pack would probably get you to 45 mpg. So are you willing to spend $3K for a maybe or just live with what you have?
     
    #3 BiomedO1, Feb 17, 2025 at 6:41 PM
    Last edited: Feb 17, 2025 at 7:07 PM
  4. Nature Boy

    Nature Boy Junior Member

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    That question of parts I used.. it does plague me as other posters have noted importance of genuine parts.
    No drag on wheels.. confirmed.
    Fan and intake cleaned and clear.
    Load testing? hmm... will check this, thx.
    Not low-rolling tires.
    Def engine filter done. Cabin filter, yes.. done. It was filthy. If the previous owner was meticuous in maintenance/upkeep of mechanical aspects, he was not concerned with exterior/interior care. Tons of decomposed leave in exterior crevices and inside was kindof a mess.. filter was bad!
    Grid charger? hmm.. will check this, Thanks!
     
    #4 Nature Boy, Feb 17, 2025 at 8:38 PM
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  5. Nature Boy

    Nature Boy Junior Member

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    Good point here.. just a shame I can't realize its full potential! :)
     
    #5 Nature Boy, Feb 17, 2025 at 8:41 PM
  6. MAX2

    MAX2 Active Member

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    Install the Hybrid Assistant app on your phone for free. Record your trip data. This will help you determine more accurately what is causing your increased fuel consumption (short trips, incorrect driving style, problems with the fuel system, or something else).

    https://attachments.priuschat.com/attachment-files/2018/01/138055_Hybrid_Assistant_Report_.pdf
     
    #6 MAX2, Feb 18, 2025 at 12:28 AM
  7. bisco

    bisco cookie crumbler

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    You can get a free 12v load test at most auto parts stores
     
    #7 bisco, Feb 18, 2025 at 1:30 PM
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  8. Prodigyplace

    Prodigyplace 2025 Camry XLE FWD

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    Using 15% ethanol fuel instead of 10% ethanol fuel affects your mpg too.
     
    #8 Prodigyplace, Feb 18, 2025 at 1:40 PM
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  9. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    You can get away with aftermarket parts in certain areas, but a Prius is relatively unforgiving about ignition, fuel injection parts and oxygen sensors.

    Honestly though a 19 year old hybrid battery can easily account for this no matter how well you've cared for the rest of the car.

    If it were mine I'd stop spending money on anything other than basic maintenance and drive it til it quit. A new battery is a big investment and it's not easy to justify doing that with such an old car; used & rebuilt batteries are rarely satisfying so I'd avoid those altogether.

    You can spend a lot of money trying to save a little gasoline with an old Prius- be careful!
     
    #9 Leadfoot J. McCoalroller, Feb 18, 2025 at 1:48 PM
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  10. Nature Boy

    Nature Boy Junior Member

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    Costo gas 98% of time. :)
     
    #10 Nature Boy, Feb 18, 2025 at 2:08 PM
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  11. BiomedO1

    BiomedO1 Senior Member

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    We're still on winter blend, so higher ethanol content. You may see a bounce back up when we move back to summer blend.
     
    #11 BiomedO1, Feb 18, 2025 at 4:30 PM
  12. Nature Boy

    Nature Boy Junior Member

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    Some very good points, thank you!
     
    #12 Nature Boy, Feb 19, 2025 at 12:14 AM
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  13. vvillovv

    vvillovv Senior Member

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    Your battery temps that DrPrius is showing are high for this time of year, even in mild Sactown winter temps. They would be normal on an 80 - 90 degree F day in summer, but even at those outside temps the pack should still show 70 - 80 degrees when you first put her into Ready Mode.
    You mentioned 10 miles a day average, it takes a while for the battery to change temps form driving or from the A/C cooling them or Heater heating them. 10 miles should only be raising battery temps 5 F. MAX ! , in 10 miles .

    Good idea to take a DrPius sceenshot after the car has been sitting all night and just after putting your Gen 2 into Ready Mode.

    Than take another screenie after your average 10 mile trip.

    Also note the - outside / ambient Temp - and how much of a difference between outside and battery.


    Mine are consistently 10 to 15 degrees F. different, summer and winter.
    I don't want to confuse you by adding a DrPrius screen shot from the Prime,
    unless you are interested.
    Primes DrPrius screens are way different with 96 lines of cell voltages as well as 96 of the bottom squares including internal resistance and cumulative delta voltage.
    It's also typically easier to read the Prime screens when it's in landscape mode instead of upright.

    I agree with @Leadfoot J. McCoalroller that installing another battery can be hard to justify and Grid Charging your hybrid battery can be a long term learning experience,

    but at 56 % lifetime down from 61 or so, if you wait to address your pack in one way or another, it's just gonna continue to get older and harder to figure out what to do with it. Or just drive it and try not to obsess about the mpgs, which is almost blasphemy to Prius owners (y)
     
    #13 vvillovv, Feb 20, 2025 at 11:28 AM
    Last edited: Feb 20, 2025 at 11:42 AM
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