PVT PLT INVOLVED IN OFF ARPT; EMER FORCED LNDG.

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I NEEDED TO BE IN ORMOND BEFORE 'EVENING CIVIL TWILIGHT' BECAUSE I MAY NOT FLY AT NIGHT. THEREFORE; I DECIDED TO GO DIRECT TO ORMOND INSTEAD OF CONTINUING THE SECOND LEG TO MELBOURNE. TO SAVE TIME I ALSO INCREASED THE PWR SETTING TO 2500 RPM (95 KTS) SO I WAS SURE TO ARRIVE AT OMN BEFORE DARK. WHEN WE WERE ABOUT 15 NM N OF TIX; I NOTICED THE FUEL GAUGES TO BELOW. BUT AS I WAS TAUGHT NOT TO RELY ON THESE GAUGES I STARTED TO CALCULATE THE ENDURANCE: THE HOBBS-METER READ PLUS/MINUS 3.1 HRS AND WITH A SIMPLE MULTIPLICATION I FIGURED I HAD BURNED 5 GALLONS PER HR TIMES 3.1 HRS EQUALS 15.5 GALLONS OF FUEL. WITH 22.5 GALLONS OF USABLE FUEL ABOARD I THOUGHT THAT I COULD FLY FOR ANOTHER 1.4 HRS (22.5 MINUS 15.5 DIVIDED BY 5 GALLONS PER HR). WHEN WE PASSED NEW SMYRNA BEACH (34J) ARPT I CONSIDERED LNDG THERE BECAUSE DAYTONA APCH TOLD ME THAT THE CEILING IN ORMOND WAS 800 FT AND THE SUN JUST DISAPPEARED. PLUS; THAT THE FUEL GAUGES WERE NOW GETTING VERY LOW; BUT I STILL DECIDED TO GO ON BECAUSE OF THE FOLLOWING REASONS: I JUST CALCULATED 1.4 HRS LEFT TO FLY. DME WAS SHOWING ONLY 10 MINS TO THE STATION (OMN VORTAC). I FIGURED THAT THE FLYING SCHOOL TO WHICH THE AIRPLANE BELONGED WOULDN'T LIKE A TELEPHONE CALL FROM ME BEING STRANDED IN NEW SMYRNA BEACH. I THOUGHT THAT MY FRIENDS WHO WOULD PICK US UP FROM ORMOND WOULDN'T' LIKE ME TO BE IN NEW SMYRNA EITHER. THE CTAF/UNICOM FREQ OF 122.7 OMN I ASKED FOR TA AND THE RWY IN USE WAS RWY 35. THAT RWY WAS ALMOST RIGHT IN FRONT OF ME SO I REACHED THE PUSH-TO- TALK BUTTON ANNOUNCE LONG FINAL FOR RWY 35. AT THAT MOMENT THE ENG STOPPED (I WAS IN A L TURN) AND IMMEDIATELY I KNEW THAT I WAS OUT OF FUEL. I MADE THE MAYDAY CALL ON 122.70 MHZ AS WELL AS 121.5 MHZ. BUT; BECAUSE I WAS ONLY 800 FT HIGH WE DIDN'T MAKE IT TO THAT FILED AND CRASH-LANDED IN THE WOODS RIGHT BEFORE THE OPEN SPOT. THE AIRPLANE WAS DECLARED 'TOTAL LOSS.' UP UNTIL THAT MOMENT WE ARRIVED AT ORM; I STILL DIDN'T KNOW WHY THERE WAS NO MORE FUEL LEFT BECAUSE I CALCULATED A HAD OVER AN HR LEFT AFTER THE CRASH. BUT; WHEN SOMEONE AT OMN ASKED ME WHAT PWR SETTING I HAD USED; IT GOT TO ME THAT I HAD MADE A MAJOR MISTAKE WITH THE ENDURANCE CALCULATION. I USED 5 GALLONS PER HR; WHICH IS ONLY FOR CRUISE PWR SETTING (2300 RPM) WHILE DURING MOST OF THE FLT I HAD USED 2400 AND 2500 RPM; WHICH USES ALMOST 7 GALLONS PER HR (7 TIMES 3.3 EQUALS 23.1 GALLONS WHICH IS GREATER THAN 22.5 USABLE GALLONS). I KNOW NOW THAT IT WAS A MISTAKE I SHOULD HAVE NEVER MADE; BUT AT THE TIME IT HAPPENED; I THOUGHT I WAS DOING EVERYTHING AS I SHOULD. THE CORRECTIVE ACTIONS IN THIS CASE WOULD HAVE BEEN: USING THE RECOMMENDED CRUISE PWR SETTING OF 2300 RPM (86 KTS). DECIDING THAT THERE WAS ENOUGH REASON TO LAND AT NEW SMYRNA BEACH AND NOT TO TAKE ANY RISK; BUT AT THAT MOMENT; I DID AND I STILL DON'T KNOW WHY.