A C182K IN CRUISE DECLARED AN EMER AND LANDED OFF ARPT DUE TO FUEL STARVATION CAUSED BY THE R TANK FILL CAP OFF.

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THE INCIDENT OCCURRED DURING THE SECOND FLT OF THE DAY. BEFORE BEGINNING THE FIRST FLT OF THE DAY; I FILLED THE ACFT FUEL TANKS. I THEN FLEW A 1 HR FLT TO PICK UP A PAX. THE INCIDENT OCCURRED APPROX 20 MINS AFTER TKOF ON THE SECOND FLT; WITH THE PAX ON BOARD. THE R FUEL FILLER CAP HAD COME LOOSE AT SOME POINT DURING THE 2 FLTS; CAUSING ALL THE FUEL TO BE SIPHONED OVERBOARD; RESULTING IN FUEL STARVATION. THE ENG QUIT; AND I WAS ABLE TO GLIDE TO A PASTURE AND LAND SAFELY. THERE WERE NO INJURIES AND NO DAMAGE TO THE ACFT. AFTER LNDG; AN A&P EXAMINED THE ACFT AND CONCLUDED THERE WAS NO DAMAGE. WE PUT 10 GALLONS OF FUEL INTO THE TANKS; AND; AFTER MOWING AND CAREFULLY INSPECTING AN ADEQUATE LENGTH OF PASTURE TO USE AS A RWY; I FLEW THE ACFT TO THE NEAREST ARPT; CHEROKEE VILLAGE; AR. BY COINCIDENCE; THE PASTURE HAD BEEN USED AS A PVT AIRSTRIP SEVERAL YRS PREVIOUSLY; AND WAS QUITE SMOOTH. AT CHEROKEE VILLAGE; I FILLED THE TANKS; AND FLEW THE ACFT TO ITS HOME BASE. THE FUEL FILLER CAPS ON THE ACFT ARE MADE BY MONARCH; AND ARE THREADED SO THEY SIMPLY SCREW INTO THE TOPS OF THE TANKS. I CANNOT IMAGINE HOW THEY COULD WORK THEIR WAY LOOSE INFLT; SO I ASSUME I FAILED TO SECURE THE R CAP AFTER FILLING THE TANKS BEFORE THE FIRST FLT. SINCE THE ACFT WAS A C182; AND SINCE THE FILLER CAPS ARE ON TOP OF THE WING; I NORMALLY TAKE ADVANTAGE OF BEING UP ON A LADDER WHILE FUELING TO INSPECT THE TOP OF THE ACFT. I DO NOT REMEMBER EITHER SECURING THE CAPS OR FAILING TO SECURE THEM.