The pilot reported that, during cruise flight, the engine "quit" while at 1500 ft above the ground. He added that he landed the airplane in a large, flat pasture, but during the landing the airplane nosed over. In a telephone conversation with the recovery specialist, he reported that the fuel tanks had not been breached during the landing, and he recovered 1.5 gallons of fuel from the right fuel tank and 2.5 gallons of fuel from the left fuel tank. The airplane was equipped with a J. P. Instruments (JPI) engine monitoring unit. The data downloaded from the unit revealed that on the accident flight, the recorded data spanned about 1 hour and 13 minutes. The data showed a sudden decrease in exhaust gas temperatures (EGT) and cylinder head temperatures (CHT) for all cylinders about the same time. For further information, see the JPI Engine Monitoring Unit data plot within the public docket for this accident. The 1967 Cessna 180 Owner's Manual states, in part: "2.5 gallons each tank unusable in normal flight maneuvers."