The pilot had made two stops without refueling before he took off on this leg of his flight. He estimated that 12 gallons of fuel was remaining when he departed Decatur, AL. While en route, he exhausted all of the fuel in the right fuel tank and switched to the left tank. As he approached the destination, he noted an indication of less than two gallons in the left tank. The pilot elected to make a precautionary landing in a grass field while he still had engine power, rather than risk total fuel exhaustion while continuing. He reported that he was fast and overshot his first intended landing field. He added power, but hit a fence at the end of the field. The aircraft crossed a road, hit a second fence, and finally came to rest in a shallow creek. According to the Hobbs meter readings, the plane had flown 4.6 hours, about the maximum endurance expected for his cruise altitudes and power settings.