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

Why Does My Prius MPG Hate Rain?

Discussion in 'Gen 2 Prius Fuel Economy' started by dstrauss, Mar 20, 2012.

  1. dstrauss

    dstrauss 2018 Honda Clarity PHEV Touring

    Joined:
    Feb 1, 2012
    54
    36
    0
    Location:
    Midland, TX
    Vehicle:
    Other Electric Vehicle
    Model:
    Touring
    We just bought a high mileage used 2009 Prius - just love it. But I am baffled by the mpg's in rain. On the flats of Nebraska, during an extensive rainstorm, my mileage dropped to 36mpg at best (60-70mph depending on intensity). Next day, on dry pavement in rolling hills of Iowa, I averaged nearly 48mpg at 70mph. Happened again the next day in Wisconsin, rain mpg dropped to low-mid 30's, back up by 10 mpg on dry pavement.

    I knew 75mph will cause a hit, but usually less than 5mpg over 70, but the rain equation is a real mystery. Is this common? :confused:
     
    #1 dstrauss, Mar 20, 2012
  2. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,073
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    No mystery here. Rain and wet pavement hurt the mileage of all cars. It takes energy to pump water out from under the tires, and it also takes energy to accelerate raindrops hitting the car. It's just basic physics.

    Tom
     
    #2 qbee42, Mar 20, 2012
  3. SageBrush

    SageBrush Senior Member

    Joined:
    Jun 4, 2008
    11,627
    2,530
    8
    Location:
    Southwest Colorado
    Vehicle:
    2012 Prius v wagon
    Model:
    Two
    Drive behind another car. All the water thrown up in the air by the tyres is work that petrol was spent to perform.

    Fixed energy costs always "appear" to affect the Prius more than other cars because of the low baseline GPM.
     
    #3 SageBrush, Mar 20, 2012
  4. dstrauss

    dstrauss 2018 Honda Clarity PHEV Touring

    Joined:
    Feb 1, 2012
    54
    36
    0
    Location:
    Midland, TX
    Vehicle:
    Other Electric Vehicle
    Model:
    Touring
    Guess I never thought of it as basic physics, coming from a 1997 GMC Suburban that got its 16 mpg whether rain, snow, shine, mountains, or just about anything else. Never better, rarely worse (around town stop and go only 12-13 mpg). Still, a 20%+ hit seems pretty heavy for rain driving. But how can I complain at more than 2x the mileage even in the rain. :)
     
    #4 dstrauss, Mar 20, 2012
  5. hyo silver

    hyo silver Awaaaaay

    Joined:
    Mar 2, 2005
    15,232
    1,562
    0
    Location:
    off into the sunset
    Vehicle:
    2004 Prius
    Model:
    N/A
    Basic for you, maybe. :)

    Would air density and humidity (not just the raindrops) also affect mileage? If the air's thicker, it follows that more energy would be required to move through it.
     
    #5 hyo silver, Mar 20, 2012
  6. cwerdna

    cwerdna Senior Member

    Joined:
    Sep 4, 2005
    12,544
    2,123
    1
    Location:
    SF Bay Area, CA
    Vehicle:
    2006 Prius
    Do some gallonage calculations as I illustrated at http://priuschat.com/forums/gen-iii...mileage-gets-worse-in-winter.html#post1411488.

    Here are some gallonage examples, for fuel used over 100 miles, notice the delta in absolute # of gallons:
    48 mpg: 2.0833 gallons
    36 mpg: 2.7778 gallons --> delta of 0.6944 gallons

    16 mpg: 6.25 gallons
    15 mpg: 6.6667 gallons -> delta of 0.4167 gallons
    13 mpg: 7.6923 gallons -> delta (from 16 mpg) of 1.4423 gallons
     
    #6 cwerdna, Mar 20, 2012
    1 person likes this.
  7. Danny Hamilton

    Danny Hamilton Active Member

    Joined:
    Apr 30, 2007
    926
    94
    0
    Location:
    Greater Chicagoland Area
    Vehicle:
    2007 Prius
    Model:
    N/A
    Also note that rain frequently comes with wind. A stiff headwind at 70+ MPH will have a noticeable effect on MPG as well.

    As cwerdna has already pointed out, the difference between 16 MPG and just a bit less means quite a bit more gasoline usage than the difference between 48 MPG and 36.
     
    #7 Danny Hamilton, Mar 20, 2012
  8. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,073
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    Yes, but here is the part that will bother you: water vapor is less dense than dry air. In other words, air gets thinner as humidity increases. This is the reason that airplanes climb poorly on humid days.

    Tom
     
    #8 qbee42, Mar 20, 2012
(You must log in or sign up to post here.)