On January 23, 2005, at 1506 Pacific standard time, a Piper PA-25-235, N6745Z, collided with a sign while the pilot was performing a forced landing on a road in Llano, California. The forced landing was precipitated by a loss of engine power. Great Western Soaring Co. was operating the airplane under the provisions 14 CFR 91. The commercial pilot; the sole occupant, was not injured; the airplane sustained substantial damage. The local glider-towing operation departed from Crystal Airport, Llano, about 1435. Visual meteorological conditions prevailed, and a flight plan had not been filed. During an interview with a National Transportation Safety Board investigator, the pilot stated that the accident flight was his 12th tow operation of the day, which equated to about 2.8 flight hours. After departing, with a glider in tow, the pilot continued to maneuver the airplane in a climbing configuration in an effort to reach an adequate altitude for the glider to release from the line. Upon reaching 8,500 feet mean sea level (msl), about 1,000 feet above ground level (agl), the airplane's engine abruptly quit. The pilot rocked the airplane's wings from side to side; a tow-plane maneuver that signals the glider pilot to release from the towline. The pilot further stated that the glider released and he maneuvered the airplane north, toward a road. While approaching the road, he noted that the surface was oriented north-south, with a downward slope in the northerly direction. With power lines located parallel to the road (on the west side), and several other lines periodically running perpendicular across the road, the pilot opted to land to the south. After the automobile traffic cleared, he completed a 180-degree turn and made a three-point landing in a southerly direction, trying to maneuver on the east side of the road, away from the power lines. On the landing roll the left wing impacted a sign located on the east side of the road, and the airplane ground looped. The airplane incurred damage to the left wing spar. The pilot added that no fuel was drained from the aircraft prior to its transport from the accident site, and no fuel was found in the unbreached fuel tank. A Safety Board investigator preformed an external inspection of the engine, noting no anomalies. Thereafter, a precursory examination was performed during which, the top spark plugs were removed and inspected; the electrodes were gray in color and slightly oval, which corresponded to normal operation according to the Champion Aviation Check-A-Plug AV-27 Chart. The engine's internal mechanical continuity was established during rotation of the crankshaft (via the propeller) and upon attainment of thumb compression in all cylinders. Magneto timing was established at 24 degrees before top dead center, which was confirmed to be the appropriate timing as indicated on the data plate affixed to the engine case; spark was obtained at each lead in the respective firing order. When manipulating the throttle into the idle position, the throttle arm did not reach the idle stop; upon further inspection, investigators noted that a portion of the cable at the throttle control had become tightened, hindering the throttle cable from reaching the stop. The belly of the airplane had a thin layer of dirt and debris consistent to the other surfaces of the airplane. Investigators drained several ounces of fluid from the gascolator, which was blue and had an odor consistent with that of aviation gasoline; the liquid was free of particulates and debris. The remaining fluid (less than a pint) was drained from the carburetor and again, investigators noted no anomalies. In an effort to assess the fuel line continuity, investigators filled the empty fuel tank with several gallons of fuel, and did not observe any external secretion of fluid throughout the length of the line. With the airplane still intact, aside from both wings separated from the fuselage, the pilot positioned himself in the cockpit and attempted to run the engine. The engine started with no difficulties noted and remained at a constant 1,200 revolutions per minute (rpm) for about 4 minutes. As he manipulated the throttle control in the forward position, the rpm increased and he subsequently performed a magneto check; no discrepancies were found. On a written log, the pilot noted the tachometer reading of 1,198.8 hours prior to departure the morning of the accident. The pilot reported refueling the airplane with full fuel in the tank at 1,200.2 hours. During the engine examination, investigators observed the tachometer to read 1,201.6 hours, equating to a total of 1.4 hours flown since the last refueling. The pilot stated that according to the airplane's Pilot Operating Handbook (POH), the airplane is equipped with a 36-gallon fuel tank located just behind the engine. After the accident, a mechanic who inspected the fuel tank said that the tank in the airplane was nonstandard, and estimated it to hold about 26 gallons. The pilot added that the airplane's fuel quantity gauge is unreliable. Upon the request of a Safety Board investigator, the pilot measured the capacity of the fuel tank by pouring a measured amount of water (in 5- and 1-gallon increments) inside the empty tank. The pilot stated the he was able to add about 26 gallons of fuel before the tank became full. In a written statement, the pilot indicated that he had an estimated 10 gallons of 100 low-lead fuel on board prior to his last departure. In a telephone conversation with a Safety Board investigator, a Pawnee expert stated that the PA-25-235 model airplanes were configured with two different types of fuel systems. One configuration consisted of two metal fuel cells located in each individual wing. The other configuration, which was that of the accident airplane, consisted of a 36-gallon tank located aft of the engine and forward of the cockpit. The 36-gallon tank located in the airplane's nose was comprised of fiberglass, and an optional rubber-like bladder was available to line the fiberglass shell. The expert was not aware of any modifications that could be used in substitution for the 36-gallon tank. He added that the unusable amount of fuel in the forward fuel tank configuration is negligible due to the short distance and direct fuel feed from the tank to the engine. During a telephone conversation with a Safety Board investigator, a Federal Aviation Administration (FAA) inspector stated that he spoke with the airplane's regular mechanic. The mechanic reported that he examined the wreckage at the accident site and observed the intact fuel tank to be dry, with no evidence of fuel contained within. He noted that the fiberglass fairing usually located under the fuel tank to protect the hopper from possible fuel contamination was missing, which made the fuel tank appear smaller than normal. After a close examination he determined that the fuel tank appeared to be the normal 36-gallon tank. The inspector further stated that a review of the airplane's logbooks revealed no entries or evidence of the fuel tank being altered or exchanged. Investigators reviewed the airplane's fueling records during the week duration prior to the accident. Several entries were made indicating the fuel quantity pumped into the airplane's tank, which included the following quantities: 32, 38, 27, and 26 gallons. Lycoming Engine Fuel Consumption Charts show the Lycoming O-540-B engine installed on the Piper PA-25-235 aircraft consumes about 23 gallons of fuel per hour at maximum power of 2575 rpm. At 2500 rpm, the aircraft consumes about 19 gallons per hour.