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Junk in the trunk; excess wight impacts on fuel economy

Discussion in 'Gen 2 Prius Fuel Economy' started by Pinback, Apr 18, 2024.

  1. Pinback

    Pinback Member

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    G'day All,
    Has anyone studied the impact of excess weight in the car vs fuel economy? I am thinking about the tools and stuff riding around in the trunk for emergencies and also what the impact might be of a trailer hitch that weighs 30lbs (class1) vs 55lbs. (class 3) The hitch is out of the wind so it is just a weight issue.
    I do cross a couple of notable hills each day to and from work.So if I were to shed say, 20 lbs of "stuff" and 20 lbs of Pinback (the hitch is for a bike rack, so it could happen) would I see a difference?

    Any old threads on this topic or experimental evidence would be appreciated.

    Pinback
     
  2. dolj

    dolj Senior Member

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    I only have an anecdote where I did a long trip with 3 adults and associated luggage and going up and down alpine passes along the way. I got the best tank average MPGs on that trip. So my personal opinion is that shedding 10 or 20 lbs from the car is not going to produce a measurable saving.

    You could always load 2 or three bags of cement (or a similar weight product) and drive a few tanks with and without and see if there is a significant difference.
     
  3. Pinback

    Pinback Member

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    I was thinking of trying something like that, loading a bunch of old equipment or lead shielding components into the cargo area and seeing at what point I would see the impact on mileage. Probably the bike(s) on the back of the car will be more of an impact due to air resistance when at freeway speed.
    I wondered about weight because the car seems to have a not so great strength to weight ratio.

    That is a very interesting anecdote about the full car on mountain roads. Best ever? Humm. I guess there is a lot to be said for regenerative braking with a battery in good shape.
    ~50 lbs of extra hitch receiver is a very small percentage of the over all weight of the car.If I recall the density of petrol at room temp is ~6lbs/gallon (3/4 of water) a that means the receiver hitch is about the same as an 8.25 gallon fill up.

    OK, I am convinced, the extra weight of the hitch will probably make no discernible difference.
    Still doing the experiment with the extra weight in the back and increasing it over time would prove interesting. I have also wondered what the engineers considered when they were specifying the battery pack. I am guessing they had to ask at what point does carrying more battery capacity quit paying off due to increased weight vs capacity? Or was it about cost or volume to fit in the car neatly?

    Pinback
     
  4. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    More weight in the car is going to affect acceleration (fuel burnt) and braking (more amperes moved around + more use of friction pads)

    So you may find less overall impact with this extra mass on a highway cruise than you would on a mail delivery route, just for example.
     
  5. Pinback

    Pinback Member

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    G'day Leadfoot,
    I think you are right, it will impact much more around town and most of my driving is on "freeways" which can feel a lot like surface street driving in LA. If my health permits me to really get back on the bike, then maybe the loss of extra personal mass will offset the weight of the receiver/hitch assembly. We'll see. And I think the additional weight is small enough to not worry about it.

    Pinback
     
  6. ebikeman

    ebikeman Junior Member

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    I was surprised at the stated weight limit in my manual. I can only have three adults or two adults and three little kids or I'm over the weight limit.

    Everything written from the manufacture says the Prius is very sensitive to weight.

    Other factors could come into play that would effect a test. Maybe do it over and over a few times under different circumstances.

    Let us know what you find.
     
  7. douglasjre

    douglasjre Senior Member

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    W a turd gen I towed a 4k lb load that was so big that even w my foot smashed to the floor The whole time I topped out at 65 mph. Fuel economy was 21. Still better than the pickup pulling the same load which would get nine 9. And you're worried about the weight of a hitch lmao
     
  8. rjparker

    rjparker Tu Humilde Sirviente

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    A full tank of gas is more weight than you are talking about and no one recommends keeping the tank less than half full for best mpg all the time.

    Obviously towing 4,000 pounds or even loading up the back with 400 pounds will impact a lot or a little.

    PS
    Being inside a 2009 Prius
    Aboard The Prius Dark Star en route to Veil Nebula

    WILL impact mental mpg