On December 16, 1995, approximately 1200 Pacific standard time, N3171B, a Cessna 170B, sustained substantial damage when it landed short of runway 16 at John Day, Oregon. The commercial pilot and his pilot-rated passenger were uninjured. Visual meteorological conditions prevailed for the local flight, which was conducted under 14 CFR 91. There was no report of the ELT actuating. The pilot stated that he was in the right seat of the airplane, and was demonstrating it to the passenger, who was a certificated flight instructor, for possible sale, when he misjudged his approach. In his written statement, the pilot stated that his takeoff climb, pattern turns and descent on final were normal. He was holding an indicated airspeed of 65 mph with very little fluctuation. The glide was good for a touchdown approximately 200 feet past the runway end, when a downdraft was encountered approximately 50 feet above the ground. The next thing he remembered was impacting the ground. The impact was on flat well-cleared, unobstructed terrain approximately 150 feet short of the threshold. He stated that light winds were favoring runway 16.