Effect of wet roads on efficiency: estimated 14.5% MPG drop

[Frank Lee]

Obviously you need to push on the dashboard to counteract the rain. :P

[Christ]

I'll drive, you push. (You'd be holding the dashboard anyway, wondering why you can only see mud where a road should be.)

[SentraSE-R]

I took my SE-R out yesterday during the first rain of the season. I was only able to achieve 37 mpg, running windshield wipers, lights, and defroster fan on my 10 mile country road loop. Conditions were moderately steady (~.5"/hr) rain with 25 mph cross winds, 55 degree F temperature, and a few puddles with 1-2" deep water.

Last week, I drove the same road in the same car and got ~50 mpg on dry roads with 10-15 mph cross winds and temperatures about 10 degres F higher. So yes, the 23% drop in mileage sounds remarkably similar to the mileage drop I experienced.

[basjoos]

My coasting distance is reduced anytime I can hear the tires "singing" on the wet road. The singing noise is produced by the tire tread displacing the water on the road when it makes contact with the road surface and then sucking it back into place as it lifts the tread off the road surface. Air is quicker at filling in the low pressure zone formed when the tread is lifted off the road. If you total up the mass of water on the road that is being displaced away from the tire's contact surface as it sets down and then sucked back into place as the tread lifts, it is considerably much more than the mass of water in the form of raindrops that is being displaced as it strikes the vehicle.

For me, a 83mpg commute on dry roads can turn into 71 mpg commute in moderate to heavy rain, average speed 55mph.

[aerohead]

I've got a table published by the EPA, which I believe has some numbers.I do remember that the SAE recommends coastdown testing be terminated if roads get wet enough to leave a tire track on the road.
I should have posted that table years ago.I'll dig it out,sorry!

[Old Mechanic]

10-20%+ depending on the amount of rain.

regards
Mech

[bgd73]

I notice nothing in the rain. not even ice storms closing in the grille and making peep holes out of the winshield..but if its foggy, maine version, cold and warm and upside down and rain and ice all at the same time...I think i am down 1%

it is the engines and their mixes, and designs. Blaming weather for a cars behalf is like trying to live in the arctic with bermuda shorts..it is rather silly.

my fathers rigs never change, no matter what, and my locale is astounding, it has everything known to mankind, even the freak giant hail. It is where I learned to get angry at most machines that start losing..ya know in the wind for one, like god wasn't ever gonna have any for your journey and ridiculous car. These statistics written, why can't it make changes in machine integrity? there are winners...so very backshelfed.

[moorecomp]

Bgd73:

Yogi Berra could not have said it better.

[aerohead]

I dug through my rat's nest,here's what I have:
Two citings,by AARP,and Robert Sikorsky( How to Get More Miles per Gallon),claim up to a 1-mpg loss due to rain.Neither gives a source for the data and both refer the reader to a table from the EPA which shows other weather related mpg data.And no reference baseline mpg is given so it's impossible to come up with a percentage difference.These data were published circa 1975,when cars were averaging about 13-mpg.
The third citing is from my textbook,Internal Combustion Engines and Air Pollution,by Obert,1973.On page 58,Fig.2-28 there is a table constructed from data taken from road tests of a Simca Aronde automobile just before and during a light rain.
The table shows mpg from 20 mph,to 60 mph,both wet and dry,and for the velocity spread,mpg suffers by 9%.
Lower mpg is attributed to increased air density,the retarding effects of tires pumping water( 832 X density of air ),and water vapor ( 100% R.H.) displacing oxygen in the charge,which softens the pressure rise of combustion ( like water injection or ADI ).
The 14% cited in the other source may be attributed to greater frontal area of modern tires,which would have greater pumping losses in the wet,and maybe also higher driving speeds since the demise of the 55-mph National speed limit.
I took a digital photo of the table and Al will post it later today when things slow down here at the copy center.

[aerohead]

Let's see if I can get this image up,