A B737-400 WAS DISPATCHED WITH THE IMPROPER FUEL LOAD IN THE CTR TANK. THE CTR TANK QUANTITY INDICATOR WAS DEFERRED AS INOP PER THE MEL. THE CTR TANK WAS NOT EMPTY WHEN THE REQUIRED 3500 LBS WERE ADDED.

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DURING PREFLT; FUELER TOLD FO THE CTR FUEL TANK QUANTITY GAUGE WAS INOP. MAINT ATTACHED AN MEL; HOWEVER; MINIMUM FUEL REQUIRED 3500 LBS FUEL TO PUT IN THE CTR TANK. THE FUEL SLIP FORM WAS COMPLETED AND A VERBAL OK WAS RECEIVED FROM MAINT CTL. THE ONLY FUELING OPTION WAS A KNOWN QUANTITY SO I ASKED MAINT CTL HOW THEY KNEW THE CTR TANK WAS EMPTY BEFORE COMMENCING FUELING WITH A KNOWN QUANTITY SINCE WE HAD NOT OBSERVED THE PUMPS ON AND LOW PRESSURE LIGHTS ILLUMINATED. THE RESPONSE WAS THEY ASSUMED IT WAS EMPTY BECAUSE A MECH HAD WORKED OVERNIGHT TO REMOVE THE INOP CTR FUEL TANK GAUGE MEL. I SAID TO VERIFY INSTEAD OF ASSUME. A MIN LATER; THE ATTENDING MECH ENTERED THE COCKPIT AND TOLD ME HE WAS SURE THE TANK HAD BEEN EMPTY BECAUSE HE HAD SPOKEN WITH THE NIGHT MECH. THE FUEL SLIP WAS SIGNED. WE DEPARTED AND BURNED FUEL OUT OF THE CTR TANK UNTIL THE LOW PRESSURE LIGHTS INDICATED THE TANK WAS EMPTY. I IMMEDIATELY PRESSED THE 'FUEL USED' BUTTON AND THE TOTAL FUEL BURNED AT THAT POINT WAS 8000 LBS; ALL OF WHICH CAME FROM THE CTR TANK. THE EXTRA 4500 LBS OF FUEL; WE WERE UNAWARE OF; PUT US ABOVE THE MAX TKOF WT FOR THE RWY AND PWR SETTING WE USED; OUR CORRESPONDING V-SPDS WERE INCORRECT; AND THIS CAUSED AN OVERWT LNDG IN ZZZ1. HAD WE EXPERIENCED AN ENG FAILURE AT TKOF; I DON'T KNOW IF WE WOULD HAVE BEEN ABLE TO SUSTAIN FLT. EITHER THE FUELER PUT FUEL INTO THE TANK WITHOUT BEING AWARE HE DID OR HE LIED TO KEEP OUT OF TROUBLE. THE MECH TOOK SOMEONE ELSE'S WORD WITHOUT DOING THE PROC AS SPECIFIED AND SIGNED HIS NAME TO AN INCORRECT FORM; POSSIBLY BECAUSE HE DIDN'T WANT A FURTHER DELAY. AS CAPT; I ACCEPTED A SIGNED FUEL SLIP AND BELIEVED SOMEONE'S WORD WHO SAID THEY DID THEIR JOB. MAINT CTL IS JUST AS CULPABLE AS THE MECH. WHO CAN I BELIEVE?