A few fuel economy questions

Discussion in 'Gen 2 Prius Fuel Economy' started by ukiltmybrutha, Nov 15, 2020.

  1. ukiltmybrutha

    ukiltmybrutha Junior Member

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    Not overly excited about my 05 Prius fuel economy. I thought it was low because it was short tripped. However, a bit longer of a trip doesn't seem to be doing much better. Fuel economy average is 35.4 mpg. This is with Mobil 1 0w-20, tires at 42 psi, and e15/88 octane (the 15 might be hurting I guess)

    I plan on taking it on back country roads with about 8 stops in 60 miles.

    Could you please tell me what a reasonable speed to drive at for testing purposes along with a speed to test it at would be so I can test further?

    Also, has anyone checked the actual mpg penalty between 88 and 87 octane? 88 is about 20 cents per gallon cheaper here but curious.

    Thanks.
     
  2. sam spade 2

    sam spade 2 Senior Member

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    First, the octane number should make NO difference.....but the ethanol content WILL, regardless of the octane that results.
    The general rule of thumb is that E10 can have a 3% negative impact, so I would expect E15 to be 4 to 4.5 %, versus E0 that is.

    Your best mileage is likely to occur when driving in the 45 to 50 MPH range without any stops or a lot of speed changes.
    That is regardless of what kind of vehicle you are in. But that isn't always possible for long stretches.

    I think your expectations might be a little too optimistic here.
    A 15 year old vehicle.....with how many hundred thousand miles on it......should not be expected to perform like new.
    That is especially true if it still has the original hybrid battery in it.
     
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  3. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    I doubt that would be detectable.

    On the other hand, there will be a detectable difference between E0/E10; you usually get about 3% better on E0 though it never actually pays for itself.

    You're still getting pretty good numbers for a 15 year old Prius, assuming original battery, wheel bearings etc.

    Most of the things you could do to restore it to "like new" performance will never pay off given expected remaining service life of the car and current low fuel prices.
     
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  4. ukiltmybrutha

    ukiltmybrutha Junior Member

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    Hello, all correct. It has 213k on the original battery. Not sure about wheel bearings etc.

    It was dealer maintained from the beginning by the previous owner.

    Thanks for helping me set expectations. My even older 02 TDI with auto gets me 47 mpg on the highway although it is a real pain to work on.

    I don't necessarily mind putting money into diminishing returns. I consider the alternative of having to pay speeding tickets and everything that comes with it.

    Thanks.
     
  5. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Yeah those don't lose efficiency until they lose compression, and usually by then the whole car is beat.

    The Prius will gradually lose fuel economy to a diminished hybrid battery, and that happens with simple age whether you use it or not.
     
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  6. fuzzy1

    fuzzy1 Senior Member

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    Please give us an idea how short your normal cold-startup trips are. The 8 stop 60 mile trip results will depend strongly on how much the engine cools during stop layovers. Hour long stops will drag it down much more than 5 minute stops.

    There is no particular test speed to try out. Drive at whatever speed is reasonable for the situation.

    From past tests by PriusChat members and their associates, Prii get their best MPG at about 15 mph according to one group, or in the 10-20 mph range according to another group. But that is unreasonably slow for most driving needs and most roads, even dangerous and illegal. So the next best answer is that your best MPG speed is the slowest speed that is otherwise reasonable for your particular situation and needs.

    Below is an MPG vs speed chart for a Gen3 Prius under ideal conditions, including a fully warmed engine that you don't get in short trips. (Bob reset his MPG meter after full warmup.) Subtract about 5 mpg for having a Gen2, then make more downward adjustments for winter cold season, E15 fuel, non-LRR tires, aging battery, any wet roads, etc. But you should still see the pattern of lower speed producing higher MPG all the way down to slow residential street or school zone speeds.

    [​IMG]
     
    #6 fuzzy1, Nov 15, 2020
    Last edited: Nov 15, 2020
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  7. ukiltmybrutha

    ukiltmybrutha Junior Member

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    Lol, the whole car is beat! You are so right!
     
  8. ukiltmybrutha

    ukiltmybrutha Junior Member

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    Normal cold startup trips are 6.7 miles in each direction. There are 6 possible stops. The longest would be 2 minutes. The others are stop signs or low congestion traffic lights. The car sits anywhere from 3 to 8 hours before making the 6.7 mile trip back. The speed limit is 45 mph and we don't exceed it willfully. Might hit up to 52 and then back off.

    Thanks for the slowest speed reasonable. I am pretty good at judging that.

    The 60 mile trip has no traffic. It should average about 55mph. Towards the end it should be about 65-70 mph on the freeway for about 20 miles.

    Appreciate the charts.
     
  9. sam spade 2

    sam spade 2 Senior Member

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    Two things use the most energy in a vehicle:
    Starting out from a dead stop.
    Bucking the wind resistance above about 10 MPH.
    The trip you describe has the worst of both things, most likely......and with a cold engine.
    Given that kind of a trip, you will continue to be disappointed and probably would have a similar disappointment with a brand new one too.
     
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  10. ukiltmybrutha

    ukiltmybrutha Junior Member

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    Pretty close. The trip begins and ends with a .25 mile or so grave/dirt road and we don't go faster than 10 mph on that. The rest is just as described.

    Sam, disappointed with what? Brand new Prius?

    Thanks.
     
  11. fuzzy1

    fuzzy1 Senior Member

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    Stop signs, traffic lights, and congestion during a single trip segment don't really count as 'stops'. Those count as ordinary 'city traffic'. The stops that I would count are the ones where you completely shut off the car and allow the engine to significantly cool down.
    That will be a much better gauge of what MPG your car can currently do, though the 70 mph portion will necessarily get lower MPG than the 55 mph portion.

    I have a crude rule of thumb, from a previous car, that a cold start costs 2 miles of fuel. This rule sort of works with my Prius too. I.e. your 6.7 mile commute from a cold start needs as much fuel as 8.7 miles of warmed-up driving on a long trip (my customary long trip is 400 miles). This rule would suggest that if it gets 35 mpg on those commutes, then it should could get closer to 44 mpg on that longer trip, if you keep down to 60 mph. The faster section will drop things a bit. But remember that this rule is far from perfect.

    For that longer trip, do reset your mpg display at the start (I forget, can a Gen2 do this?), and let us know what mpg you get at the end.
     
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