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Strange load (cargo weight) and fuel efficiency relation!

Discussion in 'Prius c Fuel Economy' started by alimoeeny, May 2, 2012.

  1. alimoeeny

    alimoeeny Junior Member

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    Hey guys,

    Last night I drove from DC to Baltimore (40.0 milles, 49 minutes, 66.3 MPG) while my PriusC was loaded with two passengers and as much stuff as we could fit in (we were moving to a new place) the car was significantly heavy, the handling felt very different, the engagement of electric and gas engine was different, it was heavy.

    Today I drove back to DC (40.9 miles, 65 minutes, 58.6 MPG) this time it was only me in the car, no cargo.

    Both times I took i495 and i95 (almost all the way highway driving), and it is a more or less flat ride, I am not sure but I don't this there is a significant slop on average either way (maybe I am wrong).

    My question is why the hell fuel efficiency was better with the heavy load?
     
    #1 alimoeeny, May 2, 2012
  2. Danny Hamilton

    Danny Hamilton Active Member

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    At a relatively constant speed the load isn't going to have a huge effect on the MPG. You'll see a bigger effect from wind resistance (drag) and change in altitude. If you are correct and there isn't much change in altitude, then my guess is that the difference is due to a tail wind on the way there and a headwind on the way back.
     
    #2 Danny Hamilton, May 2, 2012
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  3. alimoeeny

    alimoeeny Junior Member

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    It was rather windy, but I am not sure what the dirrection was,

    what surprises me most is that the road is not completely flat, so when i was going up-heel the econo indicator (or what ever its name is) often went to the orange zone because of the heavy load and I though that should affect the MPG.
     
    #3 alimoeeny, May 2, 2012
  4. seilerts

    seilerts Battery Curmudgeon

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    There is extremely little difference in actual consumption between 58 and 66 mpg. It could be as simple as having the car fully warmed up before you reset the gauge last night, vs. having to warm up the car this morning following a cold soak.

    Wind also has a significant negative impact on MPG.
     
    #4 seilerts, May 2, 2012
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  5. F8L

    F8L Protecting Habitat & AG Lands

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    With such a small sample size there is nothing you can really say about this idea. I would suggest downloading an App like "My Tracks" for your cell phone and tracking your trip. In this way you will have elevation and speed data. You will need to track the wind using weather websites. Then you can create nice little graphs like I do for my commute. Someday they will fix the elevation axis so you don't have to multiply by 10. :(

    Multiply Elevation axis (left) by 10. Total elevation is 1,200ft. ASL.
    [​IMG]
     
    #5 F8L, May 3, 2012
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  6. Danny Hamilton

    Danny Hamilton Active Member

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    I'm not nearly as surprised by the cargo weight having little effect on MPG as I am by the effect of your speed on MPG! If your numbers are correct, you were traveling at an average speed of 49 MPH on the way there and only 37.75 MPH on the way back? Did you take a different route, or make some stops on the way back? Either one would probably have an effect on MPG. Hard to picture you driving 37 MPH on I-95 and I-495 and surviving the trip.

    Also, as seilerts points out, the difference between 66.3 MPG and 58.6 MPG over 40 miles is about 0.0946 gallons (about 1.5 cups). This could be accounted for by many different variables. Was it sunnier on the way back? Perhaps the climate control was working harder. Did you switch from a tailwind to a headwind? Perhaps drag accounts for the difference. Did you turn the car off at all on the trip back? You burned extra fuel during the warm up cycle. Did you encounter heavy traffic? Perhaps the car was burning fuel to keep the HV battery charged. Etc.

    Note that if gasoline is around $4.00 per gallon, that 1.5 cups works out to be somewhere around $0.38
     
    #6 Danny Hamilton, May 3, 2012
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