The pilot was transporting the airplane from during a multi-leg trip and was on the second to last day of the trip. Prior to departure, he purchased fuel from a self-service pump and topped off both wing fuel tanks and checked that each fuel cap was properly secured. He calculated he had enough fuel for the 3.5 hour flight, plus a 40 to 50 minutes reserve, and departed. About 3 hrs. and 16 minutes into the flight, the engine lost total power. The pilot attempted to reach a nearby airport but made a forced landing about a mile short of the runway in trees, which resulted in substantial damage to the airplane’s right wing and tail section. Postaccident examination of the airplane revealed the left wing fuel cap was missing, and fuel streaks were observed aft of the cap toward the trailing edge of the wing. The left wing fuel tank was intact and empty of fuel. The right wing fuel cap was secure, and the tank was absent of fuel. The pilot stated that he did not have any issues with the fuel cap properly securing prior to the accident flight. The left fuel cap was never located. Based on available information, it is likely that the cap was not properly installed, and that it separated from the airplane at some point during the flight. Fuel was subsequently siphoned from the tank(s), and as a result, the engine lost power sooner than the pilot calculated due to fuel exhaustion.