ATC DECLARED EMER WHEN C340 ASKED FOR DIVERSION TO ALTERNATE BECAUSE OF QUESTIONABLE FUEL TO MAKE DEST.

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BEFORE DEPARTING HYANNIS; I OBTAINED A WX BRIEFING (WILLIAMSPORT); WHICH INDICATED A CEILING OF 4000 FT FOR MY RTE DOWN THE EAST COAST FOR THE APPROX 1 HR FLT. A CHANCE OF LIGHT RAIN SHOWERS WAS ALSO NOTED; AND WOULD INCREASE LATER THAT EVENING. VFR WAS NO PROB AND THAT WAS THE OPTION CHOSEN. FUEL WAS ADDED TO COVER THE FLT (EST: 1 HR AND 15 MINS); WITH SUFFICIENT VFR RESERVES. TKOF AND ENRTE; OVERWATER AND LAND WERE UNEVENTFUL UNTIL THE BRIDGEPORT; CT; AREA. PROCEEDING W; INLAND OF THE BRIDGEPORT ARPT; I ENCOUNTERED LIGHT RAIN SHOWERS (PREDICTED FOR LATER THAT EVENING). VISIBILITY WAS REDUCED TO AN UNACCEPTABLE LEVEL FOR ME (MVFR); AND I ATTEMPTED TO REACH NY APCH AFTER RETURNING TO VFR CONDITIONS. APPROX 20 MINS WAS SPENT SWITCHING FREQS AND BEING VECTORED UNTIL A CLRNC WAS OBTAINED AND EXECUTED (20 MINS WAS THE APPROX TIME NECESSARY TO HAVE REACHED CDW FROM THAT POS; VFR). I WAS VECTORED AROUND; THEN CLBED TO 5000 FT; VECTORED AGAIN; THEN UP TO 6000 FT AND THEN TO FLT CLRNC RTE (DIRECT CARMEL-NYACK-SPARTA-CDW). THAT WOULD HAVE TAKEN ME OUT TO WESTERN NJ AND THEN BACK E FOR APPROX ANOTHER 30 MINS AIR TIME. AT THE SPD AND FUEL FLOW BEING USED; THE TIME AND DISTANCE WOULD HAVE PUT ME INTO MY RESERVES. AT THAT POINT; I HAD NO IDEA HOW BADLY THE WX HAD DETERIORATED; AT CDW; AND WHETHER THE LOC APCH WOULD HAVE BEEN SUCCESSFUL OR AN ALTERNATE REQUIRED (CDW WX; AT THAT TIME; WAS VFR). MY ALTERNATE WOULD HAVE BEEN HPN; FROM WHICH I WAS; AT THAT TIME; 10 MINS DISTANT. MY FUEL FLOW COMPUTER ADVISED ME THAT I HAD 22.9 GALS REMAINING (MORE THAN ENOUGH FOR DIRECT VFR). I ADVISED CTL THAT FUEL WAS OF CONCERN AND REQUESTED DEV TO HPN. HE INQUIRED AS TO MY GALS REMAINING; AND WAS TOLD 22.9 GALS. I WAS CLRED DIRECT TO HPN; IMMEDIATE DSCNT TO 2000 FT FROM 6000 FT; AND TO INTERCEPT THE ILS FOR AN UNEVENTFUL LNDG. TWR ADVISED ME THAT EQUIP WOULD BE WAITING. I DID NOT DECLARE AN EMER; NOR DID MY FUEL LEVEL; AT THAT TIME; WARRANT A DECLARATION. THAT WAS EXPLAINED TO THE 'IN-CHARGE' ON THE GND. I CAN ONLY ASSUME THAT THE CTLR; IN DEALING WITH TURBINES AND JETS; WAS UNFAMILIAR WITH THE FUEL REQUIREMENTS OF MY TWIN CESSNA GASOLINE BURNER AND ASSUMED THAT AN EMER DID ACTUALLY EXIST.I MUST SAY THAT THE SVC WAS EXCELLENT! IN RETROSPECT; MY PROB WAS THAT I DEPENDED; MORE THAN I SHOULD HAVE; ON THE ACCURACY OF THE WX RPT; ALTHOUGH I DEPARTED SHORTLY AFTER HAVING RECEIVED THE ENRTE AND TERMINAL CURRENT CONDITIONS! SECONDLY; I SHOULD HAVE LOADED EXCESS FUEL; EVEN THOUGH IMC WAS A REMOTE POSSIBILITY. ADDITIONAL FUEL WOULD HAVE ALLOWED ME TO CONTINUE THROUGH THE BAND OF RAIN SHOWERS TO VFR AT THE CDW AREA. (I SPOKE WITH CALDWELL TWR WHILE ON THE GND AT WHITE PLAINS.) HAD I NOT DEVIATED; I WOULD HAVE BROKEN THROUGH; CANCELED IFR AND PROCEEDED VFR TO CDW; ALL WITHOUT INCIDENT. HOWEVER; THAT WAS A CHANCE I COULD NOT TAKE.