PLT OF SMA MISCALCULATES FUEL NEEDS FOR HIS TRIP BY 1 HR. RE-EXAMINES FUEL USE INFLT AND DECIDES TO DIVERT TO REFUEL.

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WAS ENRTE SGF (HOME BASE) TO ALASKA. DEPARTED SD01 VFR. ENCOUNTERED LOWERING CLOUDS VICINITY GILLETTE; WY; AND FILED IFR DIRECT LEWISTOWN; MT; FOR FUEL STOP. WHILE PASSING N OF BIL; BECAME AWARE OF FUEL GAUGES SHOWING LOWER THAN EXPECTED. HAVE FLOWN THIS PLANE FOR 10 YRS AND KNOW FUEL GAUGES ARE FAIRLY ACCURATE BELOW 3/4 LEVEL. HAD PLANNED FOR FLT TO LEWISTOWN PLUS 1 HR RESERVE. DEDUCED FROM FUEL GAUGES WOULD HAVE LITTLE OR NO RESERVE UPON REACHING LEWISTOWN. IMMEDIATELY CALLED ATC AND DIVERTED TO BIL. DUE TO FUEL UNCERTAINTY; ASKED BIL APCH TO VECTOR US DIRECTLY TO ARPT; RATHER THAN OUT FOR NORMAL ILS APCH. CTLR BROUGHT US DIRECTLY IN TO RWY 16. CTLR WAS VERY HELPFUL; REASSURING; PROFESSIONAL. BROKE OUT OF BROKEN LAYER AT APPROX 600 FT AGL; ON CTRLINE OF RWY 16. LANDED NORMALLY AND TAXIED TO RAMP. TRIED TO INSPECT FUEL REMAINING BUT NEW; RESTR FUEL PORT WOULD NOT ALLOW ENTRY OF INSPECTION MIRROR. HAVE REVIEWED FLT PLANNING; FLT TIME; ESTIMATED FUEL ON BOARD (USE DIPSTICK TO MEASURE); BELIEVE I SIMPLY MISCALCULATED FUEL BY 1 HR. BEEN FLYING 31 YRS; NO EXCUSE; BUT FIRST TIME TO HAPPEN TO ME.