According to the pilot, he departed with full fuel (75 gallons) in the airplane, for the local banner towing flight. He flew for about 3 hours and 30 minutes, and then initiated a return to the airport. During the return flight, at an altitude of approximately 1,000 feet, the airplane's engine lost power. Just prior to the power loss, while the airplane was in a climb, the pilot noted that the fuel gauge indicated 30 gallons of fuel remained. The pilot released the banner and performed a forced landing to a road. Examination of the airplane by a Federal Aviation (FAA) inspector revealed approximately 8-10 ounces of fuel in the single main fuel tank. The main fuel feed line at the bottom of the fuel tank fuel valve contained a "few ounces" of fuel. The fuel tank was filled with 30 gallons of fuel to test the accuracy of the fuel gauge, which read 33 gallons of fuel. The engine was test run on the airframe. It started and ran at full power with no anomalies noted. According to the Piper PA-25 Pilot Operating Handbook, the airplane's engine burned an average of 14-16 gallons of fuel per hour, at a power setting of 24 inches of manifold pressure and 2400 RPM. The fueler who fueled the airplane stated he filled the airplane to a capacity of 68 gallons of fuel, which is what the pilot specifically requested.