B757 FLT CREW HAS A FUEL QUANTITY DISCREPANCY AND DIVERTS FOR LNDG.

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I WAS TO GET A CHK RIDE FOR MEXICO CITY. ACFT HAD 2 MEL'S: FUEL TANK QUANTITY INDICATION SYS/FLT DECK/R MAIN TANK; AND PRESSURE FUEL SYS. THE FUELING AGENT BROUGHT THE FUEL SVC RECORD TO THE COCKPIT AND DISCUSSED THE FUEL LOADING AND VERIFICATION WITH THE LINE CHK CAPT; WHO DISCUSSED IT WITH ME. PUSHBACK; START; AND TAXI WERE NORMAL. ON TKOF; I NOTICED A HVY L WING CONDITION; REQUIRING ABOUT 3 DEGS R WING DOWN INPUT TO REMAIN WINGS LEVEL. AFTER LEVELOFF AT CRUISE ALT; WE ASKED A FLT ATTENDANT TO SEE IF THERE WERE ANY SPOILER PANELS EXTENDED. HE RPTED THAT THE SECOND PANEL FROM THE FUSELAGE; ON BOTH SIDES; WERE EXTENDED; THE SAME AMOUNT. I ASKED THE LCA TO GO BACK TO VISUALLY CONFIRM THIS. HE DID. WE CONTACTED MAINT AND DISPATCH TO DISCUSS THE ISSUE; BELIEVING WE HAD A FLT CTL PROB. LATER; A FUEL CONFIGN EICAS MESSAGE APPEARED. I LOOKED AT THE FUEL PANEL AND NOTICED A 4.8 INDICATION ON THE PREVIOUSLY BLANK R MAIN TANK READOUT. IT THEN BLANKED OUT. WE DISCUSSED THE POSSIBILITY OF BEING SHORT ALMOST 10000 LBS OF FUEL AT MEXICO CITY; WHERE TERRAIN AND ADVERSE WX WERE EXPECTED; AND THE FACT THE ACFT WAS OUT OF TRIM AND WOULD PRESENT UNKNOWN PERFORMANCE PENALTIES. WE AGAIN CONTACTED DISPATCH AND SAID WE THOUGHT IT PRUDENT TO DIVERT TO ATL. WE COORD WITH ATC TO CHANGE OUR DEST; TO GET RWY 27R FOR LNDG (THE LONGEST) AND ASKED TO HAVE THE CFR EQUIP STANDING BY. A VISUAL APCH; LNDG; AND ROLLOUT WERE NORMAL -- NO ASSISTANCE REQUIRED. WE TAXIED TO THE GATE AND WERE MET BY MAINT.