MD80 FLT PERFORMANCE INDICATES WT MUCH GREATER THAN SHOWN ON WT MANIFEST. LOAD AUDIT AT DEST SHOWED APPROX 12000 LBS MORE CARGO ON BOARD THAN INDICATED.

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DEPARTED IAH RWY 9; CLOSED OUT NORMALLY. DURING TKOF ROLL AT ROTATION; ACFT WOULD NOT ROTATE. VERY HVY. EXTRA LONG TKOF ROLL. DURING CLEANUP; ACFT SLUGGISH; ALPHA SPD DISPLAYED CLEAN UNTIL 255 KTS (3000 FT MSL LEVEL UNACCELERATED FLT). THIS INDICATED A WT AROUND 158000 LBS. CLOSEOUT WT 139800 LBS. CLBOUT WAS UNEVENTFUL BUT SLOW. CONTACTED DISPATCH TO VERIFY CLOSEOUT AND CHANGED CRUISE ALT TO REFLECT POSSIBLE INCREASE IN WT. ACFT PERFORMANCE WAS CONSISTENT WITH OVERWT ACFT. DISPATCH REPLOTTED FUEL BURN FOR LOWER ALT/HIGHER WT. REQUESTED AUDIT AT ORD. STILL OVERBURNED THAT PROJECTION BY OVER 10000 LBS. ASSIGNED RWY 27R ORD; REQUESTED RWY 27L FROM ATC; CHKED LNDG AND GO AROUND WTS; LOOKED AT OVERWT LNDG CHKLIST AND DECIDED ON A FLAPS 40 LNDG. FO WAS PF. UPON SLOWING ALPHA SPD DISPLAYED 240 KTS CLEAN. SET BUGS FOR A 138000 LB LNDG WT. POWER REQUIRED FOR STABILIZED APCH 1.42 EPR; FLAPS 40 FOR BOTH ENGS. WE WERE NOW CONVINCED THAT THE AIRPLANE WAS HVY AND A CLOSEOUT ERROR HAD OCCURRED. TRIM AT 8.0 ANU. TOUCHDOWN WAS VERY GENTLE AND ROLLED ALMOST TO THE END. AUDIT PERFORMED. FUEL AND PAX LOAD WAS SPOT ON. FREIGHT WAS LISTED AT 198 LBS; I BELIEVE. THEY UNLOADED 13 CRATES OUT OF THE MID CARGO; THE FIRST OF WHICH WAS MARKED 890 LBS. ASSUMING 900 LBS EACH THAT COMES TO ALMOST 12000 LBS DIFFERENT THAN RPTED; APPROX 6000 LBS OVER MAX ZFW. WROTE UP POSSIBLE OVERWT LNDG; MAINT INSPECTED AND SIGNED OFF. FO DID THOROUGH WALK AROUND; ALL LOOKED GOOD. FLEW ACFT TO LGA AND IT FLEW NORMALLY FOR THE CLOSEOUT WT. POTENTIAL DEADLY ERROR. GOOD AIRMANSHIP ON PART OF THE FO PREVENTED DISASTER. STATION MANAGER IN IAH SHOULD BE HELD ACCOUNTABLE.