Plastic front air deflectors - Necessary and Effective?

I see even large pickup trucks sporting factory equipped front air deflectors. This suprises me as these vehicles are bought for their ground clearance and ruggedness. A plastic air dam, or whatever term applies
diminishes both. I cannot believe any air resistance reduction would be significant and they do not appear to have any effect on radiator air flow either. Why? Is it all cosmetic?
On cars, they don't make sense either. Today's cars have insufficient ground clearance to begin with. Some models have as little as 4 to 4.5 inches!
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George Orwell wrote:

Those air dams make quite a difference in fuel economy keeping the air from funneling under. The owners manual for my truck has an air dam removal for off roading procedure.
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The answer is yes they are necessary. I once had an oval track car that the nose was sloped on the front and would not cool. Someone told me to put a defector just in front of the radiator on the bottom and it would cool. They said that was what the automotive manufacturers were doing (that was in 1982 or 83). I tried it and my temperature dropped about 20 degrees.
Doug

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Hell no, they don't serve any purpose other than to hang up on a parking curb because your stupid, fat, wife pulled over one and ripped the air dam off.
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