Following a complete loss of engine power, the Cessna 120 collided with trees, during an attempted return to the departure airport. According to the pilot, he had not visually checked the fuel quantity in the tanks prior to departure; however, the right fuel tank gauge indicated 1/4 full. The pilot also stated that the fuel gauges were unreliable. When asked if he had used a checklist, he offered that he "didn't use a written checklist, but had gone through a mental one prior to takeoff." The pilot normally flew a Cessna 180 where the middle position of the fuel selector was used for both tanks. On the Cessna 120, however; the middle position was for right tank only. No evidence of preimpact mechanical failures were discovered. Fuel was present in both fuel tanks, the primer was unlocked, and the left fuel gauge indicated 3/8 full. The fuel selector was in the "off" position; however, the mechanic first to respond to the accident site could not recall if he turned the fuel selector "off," or if it was already in that position. According to the Cessna 120 Owners Manual a pilot should "set fuel tank selector to the fullest tank for takeoff," and "not takeoff on less than 1/4 tank."