A PA28-180 PLT EXPERIENCED FUEL EXHAUSTION 3 PT 5 NM SHORT OF ZZZ; WHICH RESULTED IN A FORCED LNDG.

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THE FLT WAS PLANNED TO BE A 400 NM FLT IN IFR/VFR CONDITIONS FROM BNA TO ZZZ. PREFLT PLANNING INCLUDED ITEMS FROM THE POH; AS WELL AS STANDARD WX BRIEFING. USING A CALCULATED GS OF 105 KTS; I ESTIMATED MY TIME ENRTE TO BE 3.8 HRS. THE FLT WAS FILED AS IFR; BUT REQUIRED NO ALTERNATE AS ZZZ WAS FORECAST TO BE VFR. USING A VALUE OF 10 GPH FUEL BURN; AND WITH FULL TANKS (48 GALS); I PLANNED ON THE ACFT BEING ABLE TO FLY FOR 4.8 HRS. I HAD A REQUIREMENT OF 4.55 HRS; SO THIS FLT WAS ALSO WITHIN THE PLANNED CAPABILITIES OF THE ACFT. TANKS WERE FILLED BY THE FBO (FUEL RECEIPT SHOWING 42 GALS) AND VISUALLY INSPECTED BY THE PLT (ME). I SET THE CLOCK/TIMER DURING TAXI. I WAS COGNIZANT OF THE NEED TO BE CONSERVATIVE WITH FUEL AND; AGAIN; CHOSE A 70% CRUISE SETTING AT 2500 RPM AND LEANED AT 8000 FT TO ACHIEVE AN EGT OF 1280-1300 DEGS. DURING THE FLT; I MONITORED THE GPS; RPTED GS TO CONFIRM THAT WINDS WERE NOT STRONGER THAN FORECAST. I OBSERVED GS OF BTWN 100 AND 108 KTS DURING THE ENTIRE FLT (EXCEPT THE CLB TO 8000 FT). I UTILIZED THE GPS ADVISORY MESSAGES TO SWITCH FUEL TANKS ENRTE. IMMEDIATELY PRIOR TO THE INCIDENT; I WAS DSNDING TO 4000 FT AND THEN CANCELLED IFR AFTER VISUALLY RECOGNIZING THE ARPT ENVIRONMENT. SHORTLY THEREAFTER; THE ENG BEGAN TO FALTER AND THE GAUGE SHOWED LOW FUEL. I THEN ENGAGED THE FUEL PUMP AND ATTEMPTED A RESTART WITHOUT SUCCESS. I THEN SWITCHED TANKS AND ATTEMPTED A RESTART WITH SUCCESS AND CONTINUED ON THE APCH. AT 2800 FT; THE ENG BEGAN TO FALTER AND I NOTED THE OTHER GAUGE ALSO SHOWED EMPTY. SEVERAL THINGS OCCURRED IN THE NEXT FEW SECONDS. I NOTED THAT THE TIME ON THE CLOCK/TIMER INDICATED 3 HRS 45 MINS (DURATION OF FLT). I NOTED THAT THERE WAS LIKELY INSUFFICIENT ALT TO GLIDE TO THE RWY. I DECLARED A MAYDAY ON 122.8 AND 122.5 ADVISING I WAS EXPERIENCING PWR FAILURE AND THAT I WAS 3.5 MI E OF ZZZ ARPT AND WOULD BE MAKING AN OFF-FIELD LNDG. I TURNED OFF THE RADIO; SO I WOULD NOT BE DISTRACTED. I BEGAN A GENTLE L BANK AND SEARCHED FOR A SUITABLE LNDG LOCATION. I SET FOR BEST GLIDE AT 70 KTS AND BEGAN AN APCH; ADDING FLAPS AS NECESSARY. I AWAKENED THE CHILDREN WHO WERE SLEEPING; AND SECURED THEM TIGHTLY. I COMPLETED THE LNDG; SHUT OFF MASTER; AND EXITED WITH PAX. I CALLED 911 FROM A CELL PHONE TO ADVISE LCL AUTHS THAT AN OFF-FIELD LNDG HAD OCCURRED AND THAT THERE WERE NO INJURIES. THE TANKS WERE INSPECTED ON THE GND; AND WERE FOUND TO BE DRY. THE QUESTION LOOMS; WHY DID THE ACFT BURN 12.8 GPH INSTEAD OF ITS NOMINAL VALUE OF 8-10 GPH? THIS IS BEING REVIEWED. ITEMS OBSERVED AND HOPEFULLY LEARNED FROM THE INCIDENT. ALWAYS BE PREPARED FOR THE POSSIBILITY OF AN EMER LNDG. ACFT PERFORMANCE DATA MAY NOT BE AS DEPICTED BY THE POH. MONITOR THE ACTUAL DATA AND USE THOSE VALUES FOR FLT PLANNING INSTEAD OF 'ARBITRARY' PUBLISHED VALUES. MONITOR FUEL GAUGES ENRTE; DESPITE THEIR NOTORIOUS INACCURACY.