The pilot was flying to the accident airport to have scheduled maintenance accomplished on the airplane's cabin heater. The airplane approached runway 29, a 2,579-foot-long, 50-foot-wide, asphalt runway. While crossing the runway threshold, the pilot felt that the "ground speed was faster" than he thought. After touching down "about one-third down" the runway, the pilot utilized maximum braking and instead of running off the end of the runway, he elected to "ground loop" the airplane. The airplane veered to the right, departed the paved portion of the runway, and slid down into a ditch approximately 20 feet from the edge of the runway, damaging the left wing spar. The pilot also reported that after exiting the airplane the windsock was varying; however, it appeared to be indicating a quartering tailwind. A Federal Aviation Administration inspector confirmed continuity to all control surfaces and damage to the outboard section of the wing spar. The recorded wind at an airport located about 15 miles north of the accident site, about the time of the accident, was variable at 3 knots, gusting to 19 knots.