General aviation pilot reported engine failure in cruise flight resulted in an off-airport landing. Pilot suggested fuel starvation as the cause.

Narrative

I flew my aircraft; from ZZZ to ZZZ1. I made fuel stops along the way.On my last leg; from ZZZ2 to ZZZ1; six nautical miles from my destination the engine quit.I made a successful landing in a field. There was no damage to myself; the aircraft or private property.The flight from ZZZ2 to ZZZ1 was near the maximum range for the aircraft; but my calculations; based upon my extensive prior experience in the aircraft; was that the flight could be conducted safely and well within the FARs (plus a comfortable margin). My initial assessment was that I had somehow miscalculated my fuel consumption; and had run out of fuel. This is the explanation I gave law enforcement when he arrived at the scene. After law enforcement person completed his investigation; and confirmed that authorities had released the aircraft; we secured it; and left the scene.The next morning I returned to the aircraft and completed a thorough inspection for damage from the landing (There was none.); and for mechanical issues that might have caused the engine failure.I removed the engine cowling; and inspected the engine: Fuel lines; exhaust manifolds; ignition; and the carburetors were inspected. The float bowls were removed; the floats were checked for buoyancy; and proper functioning in the bowl; the jets were found to be clean also. Oil level was also normal.During the inspection of the aircraft; the fuel tanks were examined. The left fuel cap was very loose; and the O-ring was found to be split. The right cap was tight and sealed.I then looked inside the tanks. The right fuel tank was empty; but the left tank had fuel. After dipping the tank; it was determined that three gallons of useable fuel remained. (I have a fuel stick calibrated for useable fuel.)Next I removed and inspected the gascolator which was clean.At this point I select the left tank to 'OFF'; and put five gallons of fuel in the right tank. I started the engine; let it warm up; then shut it off to check for leaks. There were none.Because of the location and situation with the aircraft; I determined it would be better to fly it to ZZZ1; and continue my inspection there.I fastened the engine cowling; and started the engine. I completed a normal run-up; including an ignition check; then did a full power run up . All indications were normal.The field was approximately ½ statute mile long; and there were no obstacles. I decided it was safe to depart; and did so uneventfully.I suspected that the left fuel cap was the cause of the engine failure; so after landing I replaced the O-ring. Next I leveled the aircraft; and secured the left fuel cap. I then removed the left fuel line from the gascolator; and attached another length of hose so that it could drain into a fuel container.After selecting the left tank 'ON'; approximately three gallons of fuel drained into the container.I re-attached the fuel line to the gascolator; checked for leaks; and filled the left and right tanks.I completed several flights (solo) over the next few days and had no additional issues.I believe the failed O-ring allowed the cap to vibrate loose; which altered the venting of the left tank; and prevented the last three gallons from reaching the engine.