Yes, the black spoiler under the bumper. It might belp direct some air through the radiator at highway speeds but, it also blocks some air from the underside of the truck that might help in cooling the standard transmission and rear end. That's pure speculation and I'm still puzzled at the mpg gain without it. I have no idea how to calculate just how much wind drag it creates. Mine is a 4X2 so it's alot closer to the ground closing up the gap between the road and the truck.
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98 2500 CC 4X2 12v 5spd 4. 10, -silencer, AFC+20, Edelbrock IAS, VDO gauges
97 Dodge Stratus 150HP
92 Harley Fatboy, Dynojet fuel management, Supertrapp 2-1