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Unread 01-23-2015, 05:44 PM   #1
unobtanium
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Rain killing mileage?

I drove to San Antonio the other day. Got 17.5-18mpg on average. I drove back a few days later, in pouring rain. I only managed mid 15's. Can rain really kill mileage like that? I mean...maybe...considering LR tires are supposed to be worth 1-2mpg, and driving through water, even a little bit before it runs off the road probably matters...but...2-3 mpg?

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Unread 01-23-2015, 10:07 PM   #2
USN_BlackWK
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Yup. But its more than just the change in rolling resistance. Your tires need to do work to clear water (tread is designed to do this) so you loose some efficiency there. Especially in standing water. For example: an aircrafts takeoff roll can be significantly increased (500-1000ft) with just a 1/4" of standing water on the runway. Enough so that on larger aircraft we re-run the "numbers" before takeoff. You also have a loss of some of your vehicles aerodynamic efficiency. Water creates parasitic drag across the surface of the vehicle. And lastly, possibly the biggest factor in your performance, is the decrease in air density decreasing your vehicles efficiency (the more dense the air, the more air your car is sucking down the intake and vise-versa). Water vapor in the atmosphere displaces the air molecules needed for combustion. Your vehicle senses the decrease in air density reducing fuel added to the combustion chamber (to maintain a consistent efficient burn rate) and therefore less performance/efficiency. Think drag racing on a humid day vs. a cool dry night. Effectively your engine has less horsepower.

With everything combined (less horsepower and more drag) your engine is forced to compensate by scheduling more fuel for a given speed and therefore increasing burn rate and decreasing gas mileage.
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Unread Yesterday, 04:05 AM   #3
unobtanium
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Solid explanation. I had not factored other things. On the way to work tonight I again averaged my usual 18+mpg. Seems it's back to its dry normal!
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Unread Yesterday, 11:37 AM   #4
nvrdone1
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man that's the best explanation Ive heard yet. Makes total sense
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Unread Yesterday, 12:07 PM   #5
T1m
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Excellent reply USN_BlackWK!

Fly Navy Indeed! I was stationed at Whidbey Island from '77 ~ '81, attached to VA-145 and the USS Ranger.
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