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MPG dropped in the middle of a trip

Discussion in 'Gen 2 Prius Fuel Economy' started by dmaxdmax, Jun 15, 2014.

  1. dmaxdmax

    dmaxdmax Junior Member

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    Last week I drove our '05 Prius/150k miles with a single passenger and a lightly loaded hatch from Charlotte NC to Myrtle Beach SC, a distance of about 200 miles. It was hot so the AC was blasting nearly the entire time. It was a combination of highway and local road driving but no start-and-stop city nor serious Interstate miles.

    For the first ~150 miles I got an average 51.5 MPG which was great - much better than i've seen in quite a while. We stopped for lunch before going the last 50 miles. We didn't get gas, we didn't change anything except add the weight of a couple of slices of pizza to the load. By the time we reached our final destination my overall MPG had dropped to 46! Not 46 for that leg of the trip but 46 overall.

    I repeat, nothing changed - not the load, nor the fuel, nor the tires, nor the AC, nor the headlights. Nothing. How is this possible?

    Thank you!
     
  2. JC91006

    JC91006 Senior Member

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    The route would contain more inclines, wind, drafting, temperature. No route can be the same

    SM-N900P ?
     
  3. wjtracy

    wjtracy Senior Member

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    Hey DMAX I've been sayin' for a while when I stop for gas on the highway I sometimes see the same step change.
    Seems to me the display has multiple steady states (in math lingo) and one day I will get to the bottom of it.
    Bottom line it's the real calc of actual gals gas purchased that matters, and that data always seems to suggest the lower MFD value is closer to the truth.
     
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  4. dmaxdmax

    dmaxdmax Junior Member

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    JC91006 - You're right of course, it won't be exactly the same and if we were talking about the difference between 51 and 49 I'd agree it had something to do with drafting and inclines etc. The drop over the first 150 miles was ~700 feet (4'/mile) and over the last 50 it was ~40 feet (1'/mile). It was not hilly, especially at the end which was all at or near sea level.

    The only bit of interstate driving came at the beginning of the trip (which should hurt milage) and the greatest number of red lights came at the end (which should help). It doesn't make sense.
     
  5. dmaxdmax

    dmaxdmax Junior Member

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    BTW - before stopping the MFD showed *every* bar slightly above 50MPG and after stopping showed *every* bar significantly below 50. It wasn't gradual and it wasn't a rounding error.
     
  6. JC91006

    JC91006 Senior Member

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    red lights do not help at all. My wife drives my kids to school daily and sees lots of red lights. She would average about 40mpg vs close to 50 with highway driving.

    SM-N900P ?
     
  7. dmaxdmax

    dmaxdmax Junior Member

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    [/QUOTE]
    red lights do not help at all. My wife drives my kids to school daily and sees lots of red lights. She would average about 40mpg vs close to 50 with highway driving.

    SM-N900P ?[/QUOTE]

    Thanks. I haven't done manual (obsessive) milage checking since the thing was a year or so old and my faulty memory tells me that stop and go was better than highway. Still, in my case there was scarcely any difference in type of driving, not enough to account for >10mpg.

    Related but I'll look for existing threads: My electric motor is definitely not getting as much use as it did in the "old days". Coming from a full stop, the ICE used to kick in at 15-20mph but now it's almost immediate. Sometimes it continues to run during an incline - it didn't in the beginning.
     
  8. Easy Rider

    Easy Rider Active Member

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    That's a slight mis-conception you have there.
    Coming to a dead stop and then starting up again uses the most energy in ANY vehicle.
    That likely was a major factor in the difference that you saw.
     
  9. Easy Rider

    Easy Rider Active Member

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    You did say that the AC was going full blast, right ?
    That could be a factor in the ICE running a bit more.

    AND has the 12 V battery ever been changed ?
    5 years seems to be good life in these models; yours might be 9 or 10.
    With the age and mileage, it is possible that your main battery is getting a little weak too.
     
  10. dmaxdmax

    dmaxdmax Junior Member

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    By greatest number I mean 6, not 60. While I understand that coming from a dead stop uses a lot of energy I assumed much of that comes from the batteries.

    In any case, I was watching the energy display and *every* bar was significantly below 50 - including roads that were essentially identical to those I was on an hour earlier when I saw above 50. There must be circuits that tell the car when to use the electric motor along with or instead of the ICE - could those be faulty?
     
  11. dmaxdmax

    dmaxdmax Junior Member

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    The 12V battery is about 6 months old. The temps were in the mid-80s the whole way so I doubt there was much difference in the AC.
     
  12. Easy Rider

    Easy Rider Active Member

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    Not likely. I think you are grasping at straws now.
    I have ALWAYS noticed a significant drop in the rolling average when I make a pit stop in the middle of a trip.
    It doesn't take too many stop/go events to drag it way down.
    And if the beginning of the trip was in the morning and then end in the afternoon, the battery efficiency goes down a bit if it gets really hot.
    I think you are looking at this backwards and should be REALLY happy that you got over 50 on part of the trip !!! :)
     
  13. dmaxdmax

    dmaxdmax Junior Member

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    Thanks Easy Rider though I'm not grasping at straws as much as I'm trying to understand what straws there may be.
     
  14. bisco

    bisco cookie crumbler

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    i think you need to set your odo and go for a drive.
     
  15. tanglefoot

    tanglefoot Whee!

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    I think it's the pizza!

    j/k There are just so many variables that affect mpg, especially with a car like the Pri. There's probably nothing wrong with the car. It could be windspeed/direction, grade, temperature, a change in driving technique after being on the road a long time, the change in traffic type (red lights), etc.

    Stop-and-go traffic usually isn't very efficient, even with a hybrid. There's about a 40% energy loss in the conversion of kinetic energy--regen braking---electrical energy----electric drive in a stop-and-go cycle. That's considerable energy loss compared with being able to maintain speed. The electrical energy from the hybrid system certainly isn't free. It's usually more efficient to use the gasoline engine for acceleration, due to the losses associated with the regen and electric drive.

    The decision algorithms of the hybrid system use many variable inputs to decide how to transfer energy, including traction batt state-of-charge, temperature, power demand and speed. The time it takes for the gasoline engine to start after rolling out from a stop is mostly dependent on power demand (how far the pedal is pressed), but there are also other factors. The decision process is very complex and seems inconsistent at first. With enough observation in a variety of driving types, it gets more understandable and predictable (although not entirely so).

    Did you switch genres of the music you were listening to part-way through the trip?
     
  16. dolj

    dolj Senior Member

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    In my experience, I'd actually say it was the opposite of you've said here. I. e. interstate driving (which should help milage) and red lights (which should hurt)

    My only thought is that maybe you did not achieve stage 4 in the engine warm up.

    While I've observed a similar phenomenon to what you described, I can't say that I've seen such a marked difference 46 vs 51.
     
  17. rcubed3r

    rcubed3r Junior Member

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    Pepperoni?
     
  18. dmaxdmax

    dmaxdmax Junior Member

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    Wait...I didn't get the pizza! I ordered the eggplant parm! That explains everything.

    Except how I went from >50mpg to <35mpg but I officially give up.
     
  19. usnavystgc

    usnavystgc Die Hard DIYer and Ebike enthusiast.

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    It's hard for us to tell you why from here but the key for me would be what happens to your mpg from here? If it stays below 35, then you have a problem. If it goes back up, then we have to think it was an anomoly.
     
  20. PosauneGuy

    PosauneGuy Member

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    Did the weather change? Strong headwinds or rain? (I've seen my mileage drop from mid-50s to mid-40s in the middle of a trip, just from headwinds.)