According to the pilot, he reviewed the weather prior to departing on the 416-nautical mile cross-country flight. The pilot fueled the airplane with 30.1 gallons of fuel the morning of the flight. About 3 miles from the destination airport, descending through 1,500 feet msl, the engine sputtered and then lost power. During a forced landing on a grassy area, the airplane struck a tree, continued down an incline, and came to rest in a small pond, sustaining substantial damage to the fuselage, both wings, and the firewall. Examination by a Federal Aviation Administration inspector revealed that the airplane's left fuel tank contained 2.5 gallons of fuel, the right fuel tank contained 0.5 gallons of fuel, and the fuel selector was in the "BOTH" position. Review of the Cessna Model 172 Owner's Manual revealed that, the maximum usable fuel capacity in all flight conditions was 36 gallons; however, unusable fuel in level flight was 0.5 gallons. At 67 percent power, and operating with a lean mixture, the airplane's fuel consumption rate was 7.6 gallons per hour but, according to the manual, Allowances for fuel reserve, headwinds, take-offs, and climb, and variations in mixture leaning technique should be made and are in addition to those shown on the charts. Other indeterminate variables such as carburetor metering-characteristics, engine and propeller conditions, and turbulence or atmosphere may account for variations of 10 percent or more in maximum range. When asked how the accident could have been prevented, the pilot stated that he should have stopped for fuel en route.