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This paper presents field test results for fuel savings by means of flat flaps attached to the base of a standard semi trailer. The flaps are constructed from a fiberglass-epoxy-resin material and have a length equal to one-quarter of the trailer-base width (about 61 cm or 2 feet). They are attached along the rear door hinge lines on either side of the trailer and along the trailer roof-line so that no gap appears at the joint between the flap and the trailer base. The flap angle is variable and can be set to 10, 13, 16, 19 or 22 degrees. Tests were conducted in May 2004 at the NASA Crows Landing Flight Facility in the northern San Joaquin Valley, California. Analysis of the data show fuel consumption savings at all flap angle settings tested, when compared to the “no flaps” condition. The most beneficial flap angle appears to be 13 degrees, for which the fuel consumption is 0.3778 ±0.0025 liters/km compared to the “no flaps” control of 0.3941 ± 0.0034 liters/km.