The pilot reported that before departing on the cross-country flight, he determined by visual inspection, that the fuel level was about 1/2-inch below the top of the filler neck on each wing fuel tank. While established in cruise flight, after about 4 hours of flight, the airplane experienced a total loss of engine power. The pilot was unable to restore engine power and a forced landing was made to a nearby pasture. Shortly after touchdown, the airplane collided with a snow-covered depression that caused the airplane to bounce. The airplane subsequently impacted the terrain in a nose low attitude, collapsing the nose landing gear. The engine firewall and right wing sustained substantial damage during the forced landing. A postaccident examination of the airplane established that the wing fuel tanks appeared to be undamaged and void of any useable fuel. During an interview, the pilot acknowledged that the loss of engine power was likely due to fuel exhaustion. He stated that he did not use the Pilot Operating Handbook procedures to lean the fuel mixture during the accident flight. He recently had to replace a burnt engine cylinder valve, so he was operating the engine at a slightly-rich fuel mixture setting to keep the cylinders from overheating. The pilot stated that the airplane departed with about 65 gallons of fuel, which he believed would provide about 5 hours of fuel endurance while maintaining an average fuel consumption rate of 13 gallons per hour. However, following the accident, the pilot acknowledged that he did not properly account for the entire 10 gallons of unusable fuel within the fuel system.