On August 17, 1997, at 2115 central daylight time, a Piper PA-24-250 airplane, N6667P, owned and operated by the pilot as a Title 14 CFR Part 91 personal flight, was substantially damaged during a forced landing, following a loss of engine power near Caldwell, Texas. The aircraft struck a pickup truck during the landing roll. The commercial pilot, sole occupant of the airplane, was not injured; however, the occupant of the pickup truck sustained minor injuries. Visual meteorological conditions prevailed for the night cross country flight, and a VFR flight plan was filed. The flight originated from El Paso, Texas, about 3 hours 49 minutes prior to the accident, with the destination of College Station, Texas. During a telephone interview conducted by the NTSB investigator-in-charge (IIC) and on the enclosed Pilot/Operator Aircraft Accident Report, the pilot reported that he had flown the aircraft from Borrego Springs, California, nonstop to El Paso, Texas, with an en route time of 3.92 hours. At El Paso he topped off the aircraft's fuel tanks to overflowing with 50.8 gallons. On earlier flights he had observed that on a completely empty tank (left), he was only able to put 28.2 gallons in the tank to the point of overflowing, "despite the fact that the book capacity was 30 gallons." He calculated the fuel consumption from Borrego to El Paso was 13 gph (3.92 hours for 50.8 gallons). He estimated endurance for 56.4 gallons would be 4.33 hours. The pilot further reported that he used the left fuel tank for takeoff and climb for a total of one hour before switching to the right fuel tank. He flew for one hour and then switched back to the left fuel tank. After the left tank "ran dry" (total of 2 hours and 1 minute), he switched back to the right tank. After approximately 48 minutes (total time of 3 hours 49 minutes), the aircraft's engine lost power. The right fuel tank quantity gauge was indicating a quarter tank of fuel remaining. The pilot also reported that he initiated a forced landing to Highway 36, and during the approach to the highway he flashed the aircraft's landing light to warn the traffic of his intention to land. All of the traffic moved out of the way except for the pickup truck which the aircraft's left wing struck. The pilot also reported that after landing he looked into the right fuel tank with a flash light, and the bottom of the fuel tank appeared to be "buckled or bunched" up against the fuel quantity float. He checked the tank the following day and the bladder was "flat." Examination of the aircraft by the FAA inspector revealed that a section of the left wing, outboard of the landing gear was separated. The inboard section of the left wing was buckled, and the fuselage was wrinkled. Examination of the right fuel tank revealed a damp fuel stain on the wing that originated from the filler cap, and there was no "bulging" in the bottom of the tank. The FAA inspector calculated that approximately "10 gallons" of fuel had siphoned out during the flight. The pilot reported that the aircraft "experienced a siphoning leak previously on [the] original departure from CLL [College Station] to ELP [El Paso] and the short flight from Bakersfield to Orange County due to a loose cap (screw down type on the right, and lever type on the left)." He had placed a "sheet of polyethylene plastic under the left cap to shim it to seal." The right tank cap was "screwed down tight. Aware of this potential problem I visually checked both tank caps several times on climb, and there was no visual [fuel] leaking from either tank."