On January 27, 2001, at about 2140 eastern standard time, a Piper PA-28-140, N3978K, registered to a private owner, operating as a 14 CFR Part 91 personal flight, experienced a reported loss of engine power in cruise flight, and crashed during a forced landing in the vicinity of Tifton, Georgia. Visual meteorological conditions prevailed and no flight plan was filed. The airplane sustained substantial damage. The commercial pilot reported minor injuries and the private pilot rated passenger sustained serious injuries. The flight originated from Savannah, Georgia, about 1 hour 50 minutes before the accident. The commercial pilot stated they departed Savannah at about 1950, with the fuel selector on the right fuel tank. They climbed to 4,500 feet heading 270 degrees magnetic and contacted Jacksonville Center for VFR flight following. At about 2135 the engine rpm decreased from 2400 rpm to idle power. He turned the boost pump on, applied carburetor heat, and switched the fuel tank to the left fuel tank. There was no change in engine rpm. He notified Jacksonville Center they had a loss of engine power and declared an emergency. Jacksonville Center pointed out Tifton Airport, and he turned towards runway 15. His approach was too high and he made a 270 degree turn. During the turn he realized he would not be able to make the runway, he initiated a forced landing to a highway but was unable to get aligned, and crashed into a marshy area. Examination of the engine assembly and accessories was conducted at Atlanta Air Salvage in Griffin , Georgia, on January 31, 2001. No anomalies were noted. The engine was started and run at 2150 rpm. The magnetos were checked and a 100 rpm drop was noted on each magneto. (For additional information see Piper Air Safety Investigation Department Single Engine Accident Report, an attachment to this report.)