The pilot reported that when he was about 6 miles from the destination airport, the engine began running very rough. The pilot turned 180° toward an alternate airport that he had just flown over and declared an emergency. He attempted to troubleshoot the engine roughness, but was unsuccessful, and the engine lost total power. Unable to reach the runway, the pilot conducted a forced landing to a rough field. During the forced landing, the airplane sustained substantial damage to the horizontal stabilizer. Postaccident examination of the airplane revealed the right fuel tank contained no usable fuel, and the left fuel tank contained about 25 gallons of fuel. The fuel selector was found in the right fuel tank position. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation. After the accident, the pilot stated that he had no memory of changing tanks during the flight.