The pilot reported that, prior to departure, he estimated that the right tip fuel tank was three-quarters full, about 14 gallons, and each main fuel tank had about 20 gallons of fuel. The pilot reported that, during cruise, he decided to use the right tip fuel tank, even though he was aware that the right tip fuel tank gauge was inoperative. He added that, after about 25-30 minutes, he performed various stall maneuvers before returning to the departure airport to land. The pilot noticed a reduction in engine power, a drop in the manifold pressure, and concluded that the airplane would not be able to return to the airport, so he continued a left-hand turn towards a farm field. The pilot began to troubleshoot for carburetor ice, but to no avail. The airplane landed in the farm field and impacted a ditch. The airplane sustained substantial damage to the engine mounts. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The pilot added that, during a postaccident examination, it was revealed that the right tip fuel tank was dry and "that the engine lost power due to fuel starvation."