On September 18, 1999, at 1336 mountain daylight time, a Cessna 152, N65602, sustained substantial damage during a forced landing at Murray, Utah. The certified flight instructor and student pilot occupants were not injured. The flight was being operated under Title 14 CFR Part 91 as an instructional flight and no flight plan was filed. Visual meteorological conditions prevailed for this local area flight which reportedly departed Salt Lake City at 1236. The flight instructor said he was giving the student pilot a mountain checkout and after about 1.6 hours of flight, the engine began to lose power. An emergency landing was conducted to a field and the aircraft struck trees during the landing roll. Damage to the propeller, left wing spar, fuselage, windscreen, lower engine cowl and nose landing gear resulted from impact with the trees. According to the pilot operating manual for the aircraft, approximately 3.5 gallons of fuel is unusable and performance information provided evidence that the average fuel consumption in cruise flight is approximately 6.0 gallons per hour. The flight instructor said he visually examined the tanks during preflight after he noted that the gauges read slightly more than 1/2 tank fuel quantity. He estimated, based on his visual examination of the tanks, that there was approximately 13 gallons of fuel aboard the aircraft at departure. The actual fuel quantity aboard at departure was unknown. Following the accident the aircraft was examined by a FAA Inspector, who drained 1.4 gallons of fuel from the aircraft. He found no evidence of fuel leaks or engine abnormalities during his examination.