Dry roads vs wet roads

Discussion in 'Gen 3 Prius Fuel Economy' started by alfon, Feb 10, 2011.

  1. alfon

    alfon Senior Member

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    Even with temps in the 37-46 F range I took a ride to Tillamook
    with a round trips of about 115 miles. The mpg guage read
    60.3 mpg for about 57 mpg true. This is with me and two other adults in the vehicle.

    The roads were dry. With wet roads a similar trip in the same
    temperature range would give me about 50 mpg true, maybe less.

    This is with 10% Ethanol regular grade gas, Ethanol is required all year long in Oregon.

    Has anyone else noticed that wet roads seem to have a negative
    effect on mpg's more than cold temps?

    alfon
     
  2. ksstathead

    ksstathead Active Member

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    Any type of moisture on the roads is a killer.
    Cold is a killer that can be mitigated to some extent.
    Kansas winds can cost me as much as either of the above.
    Etc.

    I have not isolated wet road impact as clearly as you, but would guess extreme cold (like 0F) has a greater impact than a wet road, unless the surface is more than good and damp.

    Now rain brings more wind resistance into play as well as the rolling resistance penalty. How wet were your roads and was it raining, also?
     
  3. alfon

    alfon Senior Member

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    lEven with temps in the high 40's to low 50's with heavy rain
    and standing water it is nearly impossible to get over 50 mpg,
    45-48 mpg is about the best I will get.

    alfon
     
  4. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Yes your car is doing a lot of work to throw up all that spray behind you, that is work done by gasoline. I suspect you slip tiny amounts in the wet, and are less efficient. Cold air with rain in it is thicker than warm air without rain. You are running wiper motors, that power comes from gasoline.
     
  5. Tink

    Tink New Member

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    Not to be too picky, but moisture aside, cold air is more dense than warm air....

     
  6. nerfer

    nerfer A young senior member

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    Technically you're correct, but I think air density isn't too big a factor, otherwise the temperature alone would explain this. I too have noticed that driving in the rain reduces mpg, but I'm thinking it's more about the road surface. Flinging the water around, maybe the road is a little stickier? Not sure, but I remember one long road trip where it was raining the whole way, I expected 50-52 mpg, but got maybe 44. My wife kept the heater on, but that wouldn't explain the whole effect, especially once the cabin was warm. It's not the only case, but the most pronounced one I remember.
     
  7. F8L

    F8L Protecting Habitat & AG Lands

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    On long trips, cold doesn't have much of an effect on MPG like it does on short daily trips. Once the car the car is warmed up you're past most of the negativity of cold temps.

    Wet road will always get you no matter the temp. The wetter the road the worse it is. A lot of energy is required to pump water off the road, through your tire treads and into the air or otherwise out of the path of your tire. That energy ultimately comes from the fuel in your tank. I've never quite figured out wet lightly wetted roads reduce efficiency but it does anyway. lol My only guess is that instead of having to pump water out of the way you are picking up and moving a smaller amount of water. Anytime you move something you are doing work and that requires energy.
     
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  8. vday

    vday Member

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    :) From my expeience it doesn't seem to make that much of a difference ;)
     
  9. kutztown46

    kutztown46 Junior Member

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    I noticed the same thing very clearly yesterday. We drove to church Sunday morning. It is a 17-mile trip. We usually get 58-60 mpg per the onboard computer. This time, the roads were still wet from a rain earlier in the morning. I got only 52 or 53 mpg.
     
  10. qdllc

    qdllc Senior Member

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    Never gave it a lot of thought.

    Cold air = lower MPG because the denser air makes you burn more gas...however you get more HP because there's more oxygen/fuel in the cylinders. This should be so regardless of fuel formulation for the season.

    I never thought rain really impacted MPG all that much. Of course, I don't have many days I'm driving in a hard enough downpour to consider the roads "wet" in a way that adds considerable resistance to a rolling tire. I'd be more inclined to blame lower air temperatures (as noted above) for the loss of MPG.
     
  11. bwilson4web

    bwilson4web BMW i3 and Model 3

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    The water splashed by the tires and picked up by the body causes a lot of drag. Next time, look at the wheel wells of cars you're passing and the spray will show how much air is picked up and tossed out the side with the spray.

    Water is just under 800 times denser than air so a little bit of the liquid stuff has a big impact.

    Bob Wilson
     
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  12. cyclopathic

    cyclopathic Senior Member

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    Any car loose MPG in rain. 1st rolling resistance goes up, 2nd all the water falling on the car and being accelerated to 60mph.
     
  13. Redpoint5

    Redpoint5 Senior Member

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    You get more horsepower at wide open throttle with colder air. You're still limiting the horsepower with the throttle, which restricts the amount of air going into the engine. If the air is a little denser, very slightly less opening of the throttle is required to achieve the same amount of power output. However this is more than offset by the increased wind resistance of having to push the car through the denser air.

    Humidity also lowers the density of air. The drier the air, the more dense it is. Elevation has a huge effect on air density.

    The most efficient driving condition would have the vehicle at a higher elevation, with high humidity, and warm temperatures.

    All that said, I haven't noticed a difference in MPG when it rains, but then again I drive to the side of the worn grooves in the road. You can feel the massive deceleration when you drop into the grooves and hydroplane a little. I notice this when I'm changing lanes.
     
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