The pilot reported that, after fueling the airplane, he did not realize that he did not replace the fuel cap. During takeoff, the fuel gauge showed that the airplane had 74% of fuel (fuel gauge indicates in percent). About 5 nm from the destination airport, the fuel gauge showed that the airplane had 10% of fuel left. He flew the airplane in a straight line to the end of the runway and reduced the engine throttle setting to idle, which set up a lower glide path than normal. When the airplane reached the field west of the runway, the airplane "ran out of fuel." The airplane touched down on the field and impacted a barb wire fence during the landing roll. A Federal Aviation Administration inspector examined the wreckage and reported that less than half a gallon of fuel was recovered from the fuel tank. The airplane sustained substantial damage to the forward fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.