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300 pounds cause 10 MPG drop?

Discussion in 'Gen 3 Prius Fuel Economy' started by JStrenk, Jul 10, 2017.

  1. JStrenk

    JStrenk Active Member

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    So I was on a long trip taking my son and his kit to the NG Armory for "training". I notice my RPM's were higher, about 3800 rpm just to keep the Prius at 70MPH. MPG was indicating 38 on the display and 36 on the Scangauge. Not too happy to say the least.

    Was it the Ohio Summer gas scourge?
    Main Battery going flat?
    12 volt battery screwing things up?

    Driving it to work each day I get around 50 MPG now with the new tires. Use to be 55-60mpg depending.

    This was a long trip, about 5 hours each way, But only 300- 400" change in elevation but less than 50' at each end. Thinking that the ups and downs would cancel out in both directions.

    About the car.

    1. 128,000 miles
    2. an intermittent oil burner. used only a pint of oil on the whole 600 mile trip.
    3. tires are 195/65R-15 Goodyear Assurance TripleTred All-Season. Gas is only $1.97/gal around these parts now so I settled on a smoother less noisy tire.

    I was thinking with the new tires, it was trying harder to maintain speed.

    However:

    Driving the same roads on the way back with just myself in the car I was back up to 48 mpg.
    Gas Mileage was in distance traveled / gas pumped.. pretty close to displayed MPG and scangauge.

    What should I monitor to see if it's
    1. Tires
    2. Main battery
    3. Engine weak
    4. 12 volt battery.

    Equipment:
    1. I have scangauge, obviously.
    2. some computer program that let me crank the engine to do a pressure test and open and close the windows and other things that was fun but can't remember the name of it. :)
    3. OBDII Scanner with data logging Actron CP9580
     
    #1 JStrenk, Jul 10, 2017
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  2. bisco

    bisco cookie crumbler

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    headwind? heat? i would run a couple tanks before panicking.
     
    #2 bisco, Jul 10, 2017
  3. Mendel Leisk

    Mendel Leisk Senior Member

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    Can you clarify: was it you, your son PLUS 300 lbs of gear? That's pretty much fully loaded.

    Then there's 70 mph, that's definitely getting up there. Look at mpg vs speed charts, there's a strong/direct/linear correlation between speed increase and mpg drop.

    AC use all the way? Don't kid yourself: there's no free lunch with AC. Someone will be along shortly to tout AC use, vs driving with windows a bit cracked. Anyway, food for thought.

    That oil consumption is concerning: check the PCV valve and EGR system (stem to stern, no small task).

    Also: tire pressures?
     
    #3 Mendel Leisk, Jul 10, 2017
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  4. fotomoto

    fotomoto Senior Member

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    R&R both fuel and emissions systems and report back.

    Also, did you change tires to non-LRR type?

    Headwind on trip. I see big swings because of the nearly constant gulf breezes.
     
    #4 fotomoto, Jul 10, 2017
  5. JStrenk

    JStrenk Active Member

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    My son weight and his gear was about 300#. but it may be less.

    Tire pressure was 40# all around. AC used both ways.

    Speed was the same both ways. Mainly at 70 mph on highway and 50 on back roads.

    Do those mileage charts for speed vs mpg assume no headwinds? Say 70 mph into a 10 mph headwind is really 80 mph?
    I would say 1/3 of the trip was into a 5 mph headwind on first part of the trip and 1/3 of the trip was with no tailwind.

    I think I made a mistake when buying the tires and they are NOT LRR. But I used the same tires for the whole trip.

    Thanks for the tip on the EGR system. I'll look into that.

    Actually, I thought 48mpg on the way back was pretty good with non LLR tires, 70 mph and AC on.

    25 mile trip into work today gave me 52 mpg. so that's constant.
     
    #5 JStrenk, Jul 11, 2017
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  6. farmecologist

    farmecologist Senior Member

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    We have done two long road trips (soon to be three) in our 2010 Prius and 70+ MPH compounded with a strong headwind absolutely killed our MPG. This was expected though.

    Also, those 80 MPH freeway speed limits through South Dakota, Wyoming, Utah, etc...are horrible. Frankly, very few vehicles ( including semi trucks ) , were actually going 80 MPH when we were driving through Wyoming. It affects the MPG of ALL vehicles.
     
    #6 farmecologist, Jul 11, 2017
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  7. MelonPrius

    MelonPrius Senior Member

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    I have a short sample size with only ~13 tanks. However, weight impacts my FE more than any other factor. I've seen a dip of 8-10 mpg when I have 300+lbs of additional weight. But even having a 125 lb passenger causes a noticeable drop in my trip mpg.

    My last fill up from yesterday, I averaged lower mpg than usual. I poured a bottle of Techron in my tank and was reading up on other snake oils until I realized that I've had a passenger in my car for a good portion of my week. IMO, that was the difference.
     
    #7 MelonPrius, Jul 14, 2017
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  8. MelonPrius

    MelonPrius Senior Member

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    I missed the bolded at first, but I understand your concern. I don't have an Ultra or Scangauge hooked up to my Prius, but isn't 3800 rpm crazy high for a Prius at 70 mph? My '06 Scion with the long in the tooth 1NZ-FE engine revved very high at highway speeds and was at 3,000 rpms at 70 mph. With a CVT and a very fuel efficient ICE, that seems very high.
     
    #8 MelonPrius, Jul 17, 2017
  9. harrysprius

    harrysprius Active Member

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    Just a note about the new tires. New tires are at their least fuel efficient point. The most efficient point is just before replacement.
     
    #9 harrysprius, Jul 17, 2017
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