AN SMA PLT RAN OUT OF FUEL WITH 6 GALLONS SHOWING IN HIS TANKS HE LANDED OFF FIELD CLOSE TO HIS DEST WITH NO RPTED DAMAGE OR INJURY.

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FUEL EXHAUSTION OCCURRED DUE TO A CALCULATION OF 8.2 GALLONS PER HR BASED ON A LEANED MIXTURE AND HIGH ALT WHEN IN FACT THE SMA FIGURED OUT TO USE 8.8 GALLONS PER HR. TOTAL FLT DURATION WAS 4.5 HRS. I LEFT MARSHALL; MN; AT XA30; FLEW TO WATERTOWN; SD; (35 MIN); LANDED TO PICK UP FRIENDS; TOOK OFF AT XB25. WX WAS CLR; COOL; AND RELATIVELY CALM. AT RAPID CITY; SD; I CHKED FUEL TANK LEVELS -- R HAND TANK INDICATED 20 GALLONS; L TANK 10 GALLONS. BASED ON THAT INFO; I CONTINUED ON TO DEST WITHOUT REFUELING SINCE THE DISTANCE TO CASPER; WY; FROM RAPID CITY WAS ONLY APPROX 165 STATUTE MI; MEANING I HAD 30 GALLONS OF FUEL INDICATED TO TRAVEL THAT DISTANCE. I HAD AN ENRTE CLB SHORTLY AFTER THAT FROM 6000 TO 10500. THIS MAY HAVE USED A LITTLE EXTRA FUEL; HOWEVER; I DON'T BELIEVE IT TO BE THAT SIGNIFICANT. 25 MI OUT; I CONTACTED CASPER APCH. SHORTLY AFTER MY R HAND FUEL TANK WAS EMPTY AND SHOWING A 4 GALLON QUANTITY. I SWITCHED TANKS; TURNED ON A FUEL PUMP AND INFORMED APCH OF A FUEL PROBLEM. THEY CHANGED MY APCH TO A STRAIGHT-IN. THEN; TO MY SURPRISE; MY L TANK WAS EMPTY; INDICATING APPROX A 2 GALLON QUANTITY. AT THAT POINT MY ENG QUIT AND I FOUND A GRAVEL ROAD TO LAND ON. I HAD DONE A COMPLETE PREFLT ON THE PLANE. THE TANKS WERE FULL TO THE TOP OF FILLER NECK. I HAD CHKED THE WX THE NIGHT BEFORE AT 2200 AND AGAIN IN THE MORNING AT 0400. THE WINDS WERE FORECAST AT 320 DEG AT 25 KTS.