A CITABRIA 7KCAB AT 3000 FT 3 MI FROM THE DIVERSION RWY THE FUEL PRESSURE BEGAN TO FLUCTUATE AND ENG RAN ROUGH. MADE OFF FIELD LNDG INCURRING DAMAGE.

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CITABRIA DEPARTED ZZZ1 AT XA20 AM EASTERN TIME ON A VFR FERRY FLT (FIRST LEG) TO ZZZ2. I PERSONALLY TOPPED THE FUEL ON THE AIRPLANE (22.3 GAL 100LL) AND SECURED THE FUEL CAPS. I OBTAINED VFR FLT FOLLOWING FROM CTR DEPARTING ZZZ1. THE FLT WAS PLANNED TO STOP AT ZZZ3 FOR MY FIRST FUEL STOP (251 NM) AND WAS UNEVENTFUL. APPROX 2 HRS 45 MINS INTO THE FLT I NOTICED THE FUEL GAUGE BOUNCING BTWN 0 AND 1/4. AS THIS WAS THE FIRST TIME FLYING THIS AIRPLANE I DECIDED TO BE CONSERVATIVE AND DIVERT TO ZZZ SAVING APPROX 10 NM AND HEADWINDS. I CONTACTED ZZZ APCH DIRECT TWR AND WAS CLRED TO LAND RWY 28; APPROX 3 NM FROM ZZZ THE FUEL PRESSURE FLUCTUATED AND I TURNED THE BOOST-PUMP ON AND RICHENED THE MIXTURE. SHORTLY THEREAFTER THE ENG STARTED RUNNING ROUGH AND I INFORMED ZZZ TWR THAT I HAD A FUEL PROB AND WOULDN'T BE ABLE TO MAKE THE ARPT. I LANDED THE AIRPLANE UNDER PARTIAL TO NO PWR IN A FARMER'S COW PASTURE. ON LNDG THE AIRPLANE HIT AN ELECTRIC FENCE WIRE WHICH BROKE THE WINDSHIELD AND TIPPED THE AIRPLANE SO THAT BOTH PROP TIPS WERE SLIGHTLY BENT. THE AIRPLANE CAME TO REST ON ALL 3 WHEELS WITHOUT FURTHER DAMAGE. I RECEIVED A SMALL CUT ON MY NOSE AND FOREHEAD. THE FUEL PROB OCCURRED AT EXACTLY 3 HRS INTO THE FLT. THE AIRPLANE HOLDS 36 GALS OF WHICH 35 GALS ARE USABLE. THE POH LISTS PERFORMANCE UNDER THE CONDITIONS TO BE: 6.8 GALS PER HR; 5.1 HR ENDURANCE; 590 SM RANGE. LYC IO-320 150 HP; FIXED PITCH PROP. I CHKED THE FUEL CAPS AFTER LNDG AND BOTH WERE SECURED PROPERLY WITH NO SIGN OF FUEL STAINS. I DID NOT SEE ANY FUEL IN EITHER TANK. THE FUEL LEVEL INDICATOR WAS STILL 'BOUNCING' AT THE TIME OF FAILURE. I REMEMBER THE SELLER STATING THAT MAINT WAS PERFORMED ON THE FUEL INJECTION SYS RECENTLY BUT DID NOT QUESTION IT AS THE AIRPLANE RAN SMOOTHLY. I BELIEVE THAT THERE IS A PROB WITH THE FUEL SYS THAT LED TO A PREMATURE; UNNOTICEABLE FUEL STARVATION SITUATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FUEL PROB OCCURRED 3 HRS INTO THE FLT AND BELIEVE IT WAS CAUSED BY MECHANICAL FAILURE. THE ACTUAL CAUSE WAS UNRESOLVED AS THE FAA CLASSIFIED THE EVENT AS AN INCIDENT AND MADE NO INVESTIGATION. THE HANDBOOK STATES THE FUEL CAPACITY WAS 36 GALS OF WHICH 35 GALS ARE USABLE AND INDICATE A FUEL BURN OF 6.8 GALS PER HR AND THE TANKS WERE FULL PRIOR TO DEP. THE FLT LASTED 3 HRS AND HEADWINDS WERE MINIMAL. SEVERAL CAUSES MAY BE PRESENT WITH THIS AIRPLANE; A DRIPPING FUEL DRAIN WAS NOTED WHEN RECOVERING THE AIRPLANE AND A POSSIBLE COLLAPSED TANK BLADDER. THE FUEL PUMP SEAL LEAKAGE WOULD ALSO BE SUSPECT BUT WITH NO INVESTIGATION THE ACTUAL CAUSE WILL NEVER BE KNOWN AND COULD BE REPEATED AGAIN.