THE PLT REPORTED THAT BEFORE TAKEOFF THE WIND WAS CALM. HOWEVER, DURING THE TAKEOFF THERE WAS A SUDDEN TAILWIND. ACCORDING TO THE PLT, HE LIFTED OFF AT 65 MPH, BUT WAS UNABLE TO CLIMB AND THE ACFT SETTLED BACK DOWN ON THE RWY. THE ACFT OVERRAN THE 1800 FT LONG-GRASS RUNWAY AND WENT DOWN AN EMBANKMENT. THE PLT REPORTED NO MECHANICAL MALFUNCTION.