PLT OF AN SMA SEL ACFT INADVERTENTLY TOOK OFF OVER GROSS WT DUE TO A MISCALCULATION AND MORE FUEL WT THAN PLANNED.

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I WAS LATE FOR A TRIP; AND I PLANNED MY TRIP WITH SOME HASTE. I CAREFULLY WEIGHED ALL PAX AND CARGO. I CALCULATED MY TKOF WT TO BE 10 LBS BELOW THE MAX TKOF WT FOR THE ACFT. I ARRIVED AT THE ARPT A LITTLE LATER THAN SCHEDULED. I PREFLTED THE AIRPLANE VERY CAREFULLY. BOTH FUEL TANKS WERE FULL. I CAREFULLY FINISHED ALL PREFLT DUTIES AND WAS CLRED FOR TKOF ON RWY 2L. I CLBED OUT AT 70-75 KIAS. THIS WAS CLOSE TO VY FOR THE AIRPLANE. I NOTICED THAT THE RATE OF CLB WAS NOT NORMAL; SO I ASKED THE TWR TO LAND. THE TWR CLRED ME TO LAND ON RWY 16. I ENTERED THE TFC PATTERN AND MADE A ROUTINE LNDG. I TAXIED TO THE RAMP AND SUSPECTED THAT I MUST BE SLIGHTLY OVERWT. I TRIED TO OFF LOAD SOME FUEL; BUT THIS WAS GOING TO TAKE SOME TIME. I THEN DISCHARGED SOME PAX. I DISCHARGED THEM BECAUSE I WANTED TO BE DARNED CERTAIN THAT I WAS OPERATING THE ACFT SAFELY AND THAT THE AIRPLANE WAS NOT OVERWT. I THEN PROCEEDED WITH THE FLT TO MY DEST; AND IT WAS MADE WITHOUT INCIDENT. LATER THAN EVENING; I CHKED MY WT CALCULATIONS; AND I DISCOVERED THAT I HAD MADE A MATHEMATICAL ERROR. I WAS OFF BY 100 LBS. 2 DAYS LATER; I LEARNED THAT THE ACFT'S FUEL TANKS WERE THE EXTENDED RANGE TYPE. THIS MEANT THAT I HAD NOT INCLUDED AN ADDITIONAL 60 LBS (10 GALLONS) IN MY CALCULATIONS. AFTER ANALYZING THIS INFORMATION; I CONCLUDED THAT I WAS ACTUALLY 150 LBS OVER WT. HAD I KNOWN THIS; I NEVER WOULD HAVE TAKEN OFF. I WAS COMPLETELY UNAWARE OF MY MISTAKE; AND I BELIEVED THAT I WAS IN COMPLETE COMPLIANCE WITH THE FARS. IN MY HASTE TO PREPARE FOR THE FLT; I MADE THE WT CALCULATION MISTAKE. HOWEVER; I IMMEDIATELY TOOK REMEDIAL ACTION BY LNDG AS SOON AS PRACTICAL AND UNLOADING SOME OF THE ACFT'S WT. I WOULD RECOMMEND THAT THERE BE A REQUIREMENT FOR ACFT WITH EXTENDED RANGE TANKS. THE FAA SHOULD REQUIRE THAT A PLACARD BE PLACED ON THE INST PANEL STATING THAT THE ACFT HAS EXTENDED RANGE TANKS AND THE TOTAL CAPACITY OF USABLE FUEL OF THOSE TANKS.