Why does mileage drop when it's raining?

Discussion in 'Gen 3 Prius Fuel Economy' started by PriusRos, May 23, 2013.

  1. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

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    It seems that my mpg gauge always drops by at least 2 mpg when it's raining, even if one goes slower because of the rain. Why is this?
     
  2. mmmodem

    mmmodem Taste Tester

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    Can you run in the rain as fast as in the dry? For same reason, water increases drag on both tires and the exterior of the car making you and the car work harder.
     
  3. fuzzy1

    fuzzy1 Senior Member

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    Pumping water out of the tire track, accelerating it to considerable speed as it is expelled, takes a considerable amount of power or energy.
     
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  4. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    You are lifting a considerable quantity of water as spray, lifting all that water takes power and power takes energy. The only source of energy available is gasoline. (Except the Prius PHV)

    You are running wiper motors and air plus rain is thinker than just air, but those are minor.

    Many claim the Prius engine likes it hot, in general rainy days are cooler, and water is a much better conductor of heat than air is. so it may not make as much power per gallon on rainy days.
     
  5. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

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    Me? I can't run regardless of whether it's raining or not! :sneaky: But if I could, I'd probably be slower in the rain because I don't want to slip and fall over.

    Seriously, everybody's responses make sense intuitively but I wasn't sure.
     
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  6. css28

    css28 Senior Member

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    Think of how it feels when you hit a long puddle--the drag on the car speed. Same thing but to a lesser extent on normal wet pavement.
     
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  7. bisco

    bisco cookie crumbler

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    man, the rain really hammered my mpg's today. and the cold. 45 degrees and pouring. that was a fast summer!:cool:
     
  8. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

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    The weather has really been crazy this year. So how much drop do you see when it's raining? I'd say I probably lose around 5 mpg.
     
  9. bisco

    bisco cookie crumbler

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    i'm looking at ev miles with the plug in. i was up to about 16 with the warmer weather, and today i only got 12.:eek:
     
  10. retired4999

    retired4999 Prius driver since 2005

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    Yea! I Just about missed it! :)
     
  11. bisco

    bisco cookie crumbler

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    but 90 on friday!:eek: not digging climate change.:D
     
  12. Feri

    Feri Active Member

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    Less friction and consequently more wheel slip.
     
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  13. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

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    Today, Thursday, it was raining and my 52-mile trip home from work was at 48.5 mpg on the display.

    Monday through Wednesday were all dry and sunny, and the display for the same trip each day was 57.9 mpg, 55.6 mpg, and 56.6 mpg respectively.

    Are others finding this much difference between rainy and fair weather trips?
     
  14. Feri

    Feri Active Member

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    Was it windy? Was there lots of water on the road? Lots of accelerating decelerating? how much tread remains on your tyres?

    That's between 8 and 10 mpg drop,
     
  15. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

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    My tires are brand new(well, 800 miles old) Michelin Energy Savers (I have documented my saga about getting these in the LRR tire and the Energy Savers threads). The rain was moderately heavy but it wasn't very windy and it wasn't a lot of water on the ground. Traffic was moderate but I was going a little slower than usual because of the rain, and I may have had to do more accelerating to get up the long hills because I didn't have as much momentum.

    Today I had a 20-mile trip to the dentist in very heavy rain (no wind though) and the reading was 51.5 mpg - but the trip was pretty much downhill most of the way. Coming back, there was less rain but I made several stops and was going back uphill, and I only achieved about 42 mpg.

    As an aside, the Energy Savers are great in the rain. There was no hydroplaning or skidding on turns at all.
     
  16. Feri

    Feri Active Member

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    As an aside, the Energy Savers are great in the rain. There was no hydroplaning or skidding on turns at all.[/quote]


    Weather can really make the MPGs unpredictable. We don't have Energy Savers here. We have Energy XM2s, one of their "Green" LRR. That's the tyre I use and I'm very happy with them for the same reasons.
     
  17. Rokeby

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    Driving in the rain a Prius, any car for that matter, becomes a pump
    forceably spraying water out from under the wheels and lifting water
    up off the road surface in the spaces in the tire treads before it is flung
    off against the wheel well.

    There is also the mater of accelerating any water that strikes and adheres
    to the car up to the velocity of the vehicle.

    Here are some simulator figures showing probale MPG losses resulting
    from light medium, and heavy standing water on the roadway.
    FWIW, In the latter case , 2,595,546 lbs (311,590 Gallons) of water is being
    lifted/sprayed 4.5 feet off the highway per hour! How about Rain

    Generally, all but the gentlest rain involves wing gusts. Thesse gusts result
    in the airflow over a Prius being delflected from the most aerodynamic
    route, moving essentially straight from the front to the back of the car.
    Instead, the airflow moves at an angle and tumbles over the side of the car,
    wrecking the smooth flow and adding considerable aerodynamic drag.

    All this takes energy, which ultimately comes out of the gas tank, which
    means reduced MPGs. It's the way things are. Theres' nothing to be done
    about it.
     
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  18. retired4999

    retired4999 Prius driver since 2005

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    Most of the time if it is raining the temp. is down 10-15 degrees, this makes the MPG down a fare bit also! So rain & temps. the double wammy! :)
     
  19. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

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    Wow, that's pretty amazing. That's a lot of work for our poor little Prii!!:cry:
     
  20. Feri

    Feri Active Member

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    Last week in rain and strong southerly winds (we live North of Melbourne) my 260 km (162 Miles) round trip to Melbourne netted 5.2L/100 Kms (45.2 US mpg). Today with a light breeze and fine sunny weather I netted 4.7 L/100 Kms (50 US mpg). My long term average is 4.8 L/100Kms
     
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