FUEL EXHAUSTION FORCED ACFT TO LAND; OFF-ARPT; 7 MI SHORT OF FINAL DEST.

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THE ACFT INVOLVED WAS A C-177B (CARDINAL) WHICH I HAD RENTED IN THE PAST. THE TRIP; VGT-KNB-PGA AND RETURN; WAS FAMILIAR TO ME (15 TIMES VGT-KNB AND RETURN; AND 5 TIMES VGT-PGA-KNB-PEA; ALL IN THE PAST 18 MONTHS.) I HAD FLOWN THE CARDINAL ON MY MOST RECENT VGT-KNB TRIP; WHICH WAS A MONTH AGO. POOR FLYING WX HAS KEPT MY TIME LOW IN THE PAST 90 DAYS. AFTER RETURNING FROM PGA; ON THE GND; IN KNB; I CHKED FUEL AND ESTIMATED 15 GALLONS ON BOARD. I DECIDED TO DIVERT TO SGU AND ADD FUEL FOR RESERVE. IN SGU I ADDED ANOTHER 10 GALLONS. THE HOBBS SHOWED 3.7 HRS ELAPSED: I BELIEVED THE EXTRA FUEL WAS MORE THAN PLENTY BASED ON 4.9 HR ENDURANCE STATED IN THE PLT OPERATING HANDBOOK. (THE TANKS WERE TOP FULL IN VGT.) THE FORCED LNDG WAS SUCCESSFUL -- THANK GOODNESS FOR GLIDER TRAINING; AND THERE WERE NO INJURIES; AND NO DAMAGE. I THINK THE DISCREPANCY BTWN THE PLT OPERATING HANDBOOK ENDURANCE AND ACTUAL ENDURANCE IS A RESULT OF THE INCREASED PWR SETTINGS ON CLB; TOO RICH A MIXTURE; AND GREATER-THAN-PLT OPERATING HANDBOOK PWR SETTINGS ON CRUISE. MY ALT WAS; OF COURSE; NOT ALWAYS; AS IN THE PLT OPERATING HANDBOOK CHART. I PLAN TO FLY THE SAME TRIP AGAIN; FUELING AT EACH STOP TO CHK CONSUMPTION; USING THE SAME PWR SETTINGS AND FIND OUT WHAT THE FUEL CONSUMPTION REALLY WAS.