Looking for a sanity check on MPG on road trip

Discussion in 'Gen 2 Prius Fuel Economy' started by dylanjcole, Mar 27, 2024.

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  1. dylanjcole

    dylanjcole New Member

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    Hello everyone! This is my first post, and I think I found the right place to write it, but please let me know if I'm doing something wrong. I am new to this forum and don't want to ruffle any feathers.

    I have a 2006 Prius with 119,000 miles. I typically get 42mpg at home. On this road trip, I have been getting only 34mpg. Here are some ways the the road trip differs from my typical driving:
    • Carrying a bike on back and have additional water/gear weight in car
    • Lots of mountain passes that fully deplete the battery on the way up
    • High elevation - about 6000ft vs 0 ft elevation
    • Typical speed 65 to 70 mph
    • Generally pretty cold weather - 28 to 45 degrees F
    • Tested with the Dr. Prius app - error code B2799
    So - I'm basically wondering if my drop is milage seems normal given these conditions, and also I'm not sure what to make of the error code B2799. I have searched around on forums but this is all pretty new to me and I'm not sure what steps I should be taking. Thanks in advance!
     
    #1 dylanjcole, Mar 27, 2024
  2. bisco

    bisco cookie crumbler

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    welcome!
    yes, it's a combination of all those things, especially the mountains

    b2799-539-code.157891
    you might need a better code reader for the sub code
     
    #2 bisco, Mar 27, 2024
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  3. fuzzy1

    fuzzy1 Senior Member

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    The bike rack, or any external cargo, costs a lot of fuel due to the increased air drag. On a Prius, even a back-mounted bike rack leaves most of the bike out in the direct windstream. The 'protected' tail zone wind shadow is much smaller than any adult bike.

    My experience in Gen3 (which can climb mountains on the ICE alone, without depleting the battery) and in non-hybrids has been that elevation alone does not cost much fuel, in fact it saves fuel from reduced air drag. But other mountain factors, such as steep downhills (too steep to glide and to much descent to adequately capture the potential regeneration), colder weather, wet or slushy roads, increased wind, etc., can be costly.
     
    #3 fuzzy1, Mar 28, 2024
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  4. dolj

    dolj Senior Member

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    Assuming you have not replaced the hybrid vehicle ECU, the other reason you could have a B2799 has to do with disconnecting the battery. In particular, the power switch's power mode was changed to IG-ON and then the battery was disconnected. If, during reconnection, the key is now not in the key slot, DTC B2799 may be output.
     
    #4 dolj, Mar 28, 2024
  5. dylanjcole

    dylanjcole New Member

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    Thank you - that makes a lot of sense. I will stop worrying about MPG.
     
    #5 dylanjcole, Mar 28, 2024
  6. dylanjcole

    dylanjcole New Member

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    That's exactly what happened! I didn't turn off the car before I disconnected the battery to reset the fuel gauge after I ran out of fuel and refilled (which worked to reset it.) I'm assuming that what I need to do now is turn off the car, disconnect the battery for a few minutes, put the key in the slot, reconnect the battery, and turn the car on? Thanks very much for your help.
     
    #6 dylanjcole, Mar 28, 2024
  7. dolj

    dolj Senior Member

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    ... or just use Dr. Prius to clear the code.
     
    #7 dolj, Mar 28, 2024
  8. dylanjcole

    dylanjcole New Member

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    That sounds much better to me! I would have thought that clearing the code would be a band-aid fix and that it would come back, but I guess if it's just an electronic thing then maybe not.
     
    #8 dylanjcole, Mar 28, 2024
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