MD80 FLT CREW NOTICES A FUEL DISCREPANCY BTWN THE FUEL GAUGES AND THE FUEL SHEET OF OVER 600 LBS.

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THIS IS THE SECOND RPT FOR 2 SIMILAR OCCURRENCES WHICH HAPPENED. PRIOR TO DEP AND AFTER RECEIVING THE FUEL SLIP; I NOTICED A DISCREPANCY BTWN WHAT WE HAD FOR TOTAL FUEL ON BOARD AS READ FROM THE GAUGES AND WHAT WAS SUPPOSED TO BE ON BOARD WHEN USING THE ARR FUEL (LBS) AND THE GALLONS ADDED (TIMES THE DENSITY) AS ANNOTATED ON THE FUEL SLIP. NOTIFIED MAINT. USED THE FUEL STICK MEASURING METHOD AND IT CONFIRMED THAT THE ACTUAL FUEL ON BOARD MATCHED WHAT WAS ON THE FUEL SLIP. THE FUEL GAUGES WERE FOUND TO BE READING 1000 LBS HIGH. 500 LBS PER WING TANK. RELEASE FUEL WAS 18744 LBS. L TANK GAUGE SHOWED 9450 LBS; R TANK SHOWED 9450 LBS AND CTR SHOWED 100 LBS. ACFT ARRIVED WITH 4744 LBS AND THE FUELER ADDED 1917 GALLONS. 1917 GALLONS TIMES THE DENSITY OF 6.81 = 13054 LBS. ACCORDING TO THE FUEL SLIP; WE HAD 4744 LBS AND 13054 LBS ADDED WHICH SHOULD EQUATE TO 17798 LBS BUT THE FUEL GAUGES READ 19000 LBS. FUEL STICK MEASURING METHOD CONFIRMED THAT WE HAD ROUGHLY 18000 LBS OF FUEL ON BOARD. THIS ACFT HAD RECENTLY UNDERGONE THE FUEL CONFIDENCE CHK. THE ACFT WAS TAKEN OTS.