The pilot reported that the flight was commenced with full fuel tanks. An endurance of over 210 minutes was anticipated with the planned fuel flow. The engine failed after three hours and the aircraft sustained damage to the nose gear and right wing during the ensuing forced landing. The pilot advised that when he subsequently dipped the fuel tanks there was no fuel remaining. The pilot had not leaned the mixture correctly, and the consequent fuel flow was greater than he had expected. On previous occasions he had operated the aircraft for shorter flight periods and had not calculated the actual fuel usage rate.