AC50 FLT CREW DEPARTS WITH ENOUGH RESERVES FOR A VFR FLT BUT ENDS UP DIVERTING DUE TO UNFORECAST WEATHER ENROUTE AND ASSOCIATED ATC VECTORS.

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COMPANY POLICY WAS NOT TAKEN INTO CONSIDERATION AND WE TOOK OFF WITH LESS THAN 90 GALLONS OF FUEL. DURING THE FLT; THE CREW WAS UNAWARE THAT THE FUEL GAUGES READ INACCURATE UNDER 50 GALLONS. THE FUEL TRUCK WAS INOP AT ZZZ AND WE DID NOT HAVE CONTRACTED PRICES FOR FUEL. THE DECISION WAS MADE BY THE PIC TO TAKE OFF AND REFUEL AT OUR NEXT STOP. RESERVES WERE ADEQUATE FOR THE FLT AND NO ALTERNATE WAS NEEDED DUE TO THE FORECAST. ABOUT 40 MI OUT A TSTM CELL HAD POPPED UP; THIS ALONG WITH UNFORECASTED WINDS GAVE US A PROB. INSTEAD OF A XWIND; WE HAD A HEADWIND; AND THIS ONLY BECAME MORE EVIDENT AS WE GOT CLOSER. IT BECAME OBVIOUS THAT ATC WANTED TO VECTOR US AND CLEARLY WE DID NOT THINK WE HAD THE FUEL FOR THAT OP. WE MADE THE DECISION TO DIVERT TO GET DOWN SAFELY IN VMC. WE GOT INDICATED LOW INTO OUR RESERVES AND WE DECIDED TO DECLARE A FUEL EMER. LNDG SAFELY; TOPPING OFF; AND TALKING TO THE COMPANY; WE LATER REALIZED THAT THE FUEL GAUGE INDICATED INCORRECTLY. TOO MUCH EMPHASIS AND TRUST WAS PUT INTO OUR FUEL GAUGE INSTEAD OF TRUSTING OUR FLT PLANNING.