1. Attachments are working again! Check out this thread for more details and to report any other bugs.

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

    Joined:
    Jun 22, 2006
    1,973
    218
    0
    Location:
    Rockville, MD
    Vehicle:
    2016 Prius
    Model:
    Four
    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?
     
    #1 PriusRos, May 23, 2013
  2. mmmodem

    mmmodem Senior Taste Tester

    Joined:
    Nov 17, 2011
    2,732
    1,703
    0
    Location:
    Bay Area, CA
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    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.
     
    #2 mmmodem, May 23, 2013
  3. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    17,105
    10,039
    90
    Location:
    Western Washington
    Vehicle:
    Other Hybrid
    Model:
    N/A
    Pumping water out of the tire track, accelerating it to considerable speed as it is expelled, takes a considerable amount of power or energy.
     
    #3 fuzzy1, May 23, 2013
    Codyroo likes this.
  4. JimboPalmer

    JimboPalmer Tsar of all the Rushers

    Joined:
    Apr 14, 2009
    12,470
    6,862
    2
    Location:
    Greenwood MS USA
    Vehicle:
    2012 Prius v wagon
    Model:
    Three
    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.
     
    #4 JimboPalmer, May 23, 2013
  5. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

    Joined:
    Jun 22, 2006
    1,973
    218
    0
    Location:
    Rockville, MD
    Vehicle:
    2016 Prius
    Model:
    Four

    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.
     
    #5 PriusRos, May 25, 2013
    mmmodem likes this.
  6. css28

    css28 Senior Member

    Joined:
    Feb 27, 2012
    1,566
    442
    3
    Location:
    Suburban Detroit
    Vehicle:
    2011 Prius
    Model:
    Three
    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.
     
    #6 css28, May 25, 2013
    Codyroo likes this.
  7. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,693
    48,945
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    man, the rain really hammered my mpg's today. and the cold. 45 degrees and pouring. that was a fast summer!:cool:
     
    #7 bisco, May 25, 2013
  8. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

    Joined:
    Jun 22, 2006
    1,973
    218
    0
    Location:
    Rockville, MD
    Vehicle:
    2016 Prius
    Model:
    Four

    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.
     
    #8 PriusRos, May 25, 2013
  9. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,693
    48,945
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    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:
     
    #9 bisco, May 25, 2013
  10. retired4999

    retired4999 Prius driver since 2005

    Joined:
    Aug 11, 2011
    2,652
    625
    15
    Location:
    Eau Claire, Wi.
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    Yea! I Just about missed it! :)
     
    #10 retired4999, May 27, 2013
  11. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,693
    48,945
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    but 90 on friday!:eek: not digging climate change.:D
     
    #11 bisco, May 27, 2013
  12. Feri

    Feri Active Member

    Joined:
    Jun 25, 2011
    761
    144
    0
    Location:
    Maldon Victoria Australia
    Vehicle:
    2010 Prius
    Model:
    III

    Less friction and consequently more wheel slip.
     
    #12 Feri, May 28, 2013
    Rupert B Puppenstein likes this.
  13. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

    Joined:
    Jun 22, 2006
    1,973
    218
    0
    Location:
    Rockville, MD
    Vehicle:
    2016 Prius
    Model:
    Four
    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?
     
    #13 PriusRos, Jun 6, 2013
  14. Feri

    Feri Active Member

    Joined:
    Jun 25, 2011
    761
    144
    0
    Location:
    Maldon Victoria Australia
    Vehicle:
    2010 Prius
    Model:
    III

    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,
     
    #14 Feri, Jun 6, 2013
  15. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

    Joined:
    Jun 22, 2006
    1,973
    218
    0
    Location:
    Rockville, MD
    Vehicle:
    2016 Prius
    Model:
    Four

    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.
     
    #15 PriusRos, Jun 7, 2013
  16. Feri

    Feri Active Member

    Joined:
    Jun 25, 2011
    761
    144
    0
    Location:
    Maldon Victoria Australia
    Vehicle:
    2010 Prius
    Model:
    III
    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.
     
    #16 Feri, Jun 8, 2013
  17. Rokeby

    Rokeby Member

    Joined:
    Jan 21, 2008
    3,033
    708
    75
    Location:
    Ballamer, Merlin
    Vehicle:
    2008 Prius
    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.
     
    #17 Rokeby, Jun 8, 2013
    PriusRos likes this.
  18. retired4999

    retired4999 Prius driver since 2005

    Joined:
    Aug 11, 2011
    2,652
    625
    15
    Location:
    Eau Claire, Wi.
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    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! :)
     
    #18 retired4999, Jun 8, 2013
  19. PriusRos

    PriusRos A Fairly Senior Member - 2016 Prius Owner

    Joined:
    Jun 22, 2006
    1,973
    218
    0
    Location:
    Rockville, MD
    Vehicle:
    2016 Prius
    Model:
    Four
    Wow, that's pretty amazing. That's a lot of work for our poor little Prii!!:cry:
     
    #19 PriusRos, Jun 8, 2013
  20. Feri

    Feri Active Member

    Joined:
    Jun 25, 2011
    761
    144
    0
    Location:
    Maldon Victoria Australia
    Vehicle:
    2010 Prius
    Model:
    III
    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
     
    #20 Feri, Jun 19, 2013
(You must log in or sign up to post here.)