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found my data
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.
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