While on a cross-country flight, the pilot elected to make an unscheduled stop at an unfamiliar airport to check the en route weather. He circled the airport and thought the wind indicated favored runway 31, but later, he found that the wind was calm. No response was received when he attempted to contact the airport UNICOM. Runway 31 sloped downhill and had a 280 ft displaced threshold. The pilot was unaware that only 1550 ft of runway was available since his sectional chart indicated that it was 1800 ft long. Also, he was unaware that most pilots used runway 13 when possible to avoid the downslope on runway 31. During the landing, he touched down near midfield. He recognized then that there might not have been sufficient runway to stop, but raised the flaps and braked hard, rather than go around, because of trees beyond the end of the runway. Despite heavy braking, the aircraft continued off the end of the runway and collided with a ditch.