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Discussion Starter · #1 ·
Hey, on my everyday drive to work and back I noticed a drop in mpg of about 15-20%, when it's raining outside or has just rained before. Is that normal? Sound crazy high to me and I've never experienced that with another car before. :confused:
 

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Drive thru many firths or fjords on the way to work?
 

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Wet roads increase tire rolling resistance and will decrease mpg but doubt it will be 15-20%.
 

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Discussion Starter · #4 ·
80% highway and not really any significant water on the road.
 

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Engine volumetric efficiency should be higher with more dense air, which should "peak shave" the effect of higher rolling resistance on a wet surface to some minor degree.

The data of lower fuel efficiency when it is raining does not seem very valid conceptually, and certainly not 15%

that's my two cents

P
 

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I don't have numbers for the aerodynamic efficiency of the XC-90 but I know of small airplanes that fly slower in the rain because the water droplets disrupt the smooth airflow over the wings.
 

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Is it possible you're driving different?
 

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I would suggest you remove all coils and plugs for an inspection. You may have high voltage leakage during wet conditions.
 

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No, I use the cruise control as much as I can. :D
assuming you are not foolish enough to use cruise control in the rain, :thumbdown: there is your answer. cruise control gets you better mpg. where are the tappet brothers when you need them?
 

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Discussion Starter · #11 ·
I don't get what you're trying to say. I said I'm using the CC at the same speed whether it's raining or not. Care to explain what's foolish about that?
 

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I don't get what you're trying to say. I said I'm using the CC at the same speed whether it's raining or not. Care to explain what's foolish about that?
no, im too lazy to explain. search for yourself about why one should not use cruise control in the rain.
 

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CC use on any kind of slippery surface can be really treacherous if the car loses traction. It will react so quickly (code for erratically) that many drivers will not be able to control the car.
 

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Discussion Starter · #14 ·
Not trying to be rude, but what the heck does that have to do with my problem from the OP anyway?
 

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I've had a small "Ahah!" moment: You may have a non-working thermostat.

If so, in dry weather the engine will still warm up pretty well because the cooling system is not oversized at all. But in rainy weather, the radiator would be cooled even more by the water droplets, perhaps to the point of cooling the engine down very low. And it just so happens, that recently while doing other maintenance work on my neighbor's 2.5T, I discovered just that ... a thermostat that was in pieces inside the housing. The engine temp gauge was still indicating OK temps ... but the thermostat was definitely not doing its job. An overcooled engine will not be very efficient.

The neighbor's car is now running "better than ever", but the thermostat replacement was just one of a number of maintenance actions taken.
 

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If you are using cruise control and are indeed driving the same way in the rain, how heavy are you on the gas? Keep in mind that if you have an AWD model, it runs in mostly FWD mode in dry weather to conserve fuel economy. When it starts needing to use the rear wheels, your fuel economy will be slightly lower.
 
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