The pilot departed on a cross-country flight from Jackman, ME to Rochester, NH and return. Prior to takeoff, the fuel tanks were topped off. During the preflight, the left fuel gauge was found to be "pinned" at the full position and was inoperative. After landing at Rochester, the pilot took off on the return flight without refueling. At that time, he noted that the operating (right) fuel gauge was indicating at least half full. While en route in the vicinity of Sebago Lake, the pilot noticed that the remaining gauge was quickly becoming low. He changed course to land at Sanford, ME. Before reaching Sanford, the engine lost power from fuel exhaustion after about 3.7 hrs of flight time. The pilot attempted to land in a small field with trees and wires at the near end, but was unable. He then aimed the aircraft between rows of trees. Subsequently, the wings collided with tree trunks at about 18 ft AGL. The pilot stated he figured the fuel consumption would be 4 to 4.3 gal/hr. The actual consumption was about 6 gal/hr.