DRIVER STATED THAT HE STARTED OFF HILL WITH LOAD, STOPPED ON LEVEL AREA & PUT TRUCK IN 3RD GEAR,DRIVER STATED THAT HE APPARENTLY DID NOT GET SHIFT ALL THE WAY IN & IT POPPED OUT GOING DOWN THE HILL. DRIVER DECIDED TO TURN BY PASS ROAD AT HIGH SPEED, ROAD SLOPE CHANGED & THE COMBO CAUSED TRUCK TO OVERTURN. AFTER INCIDENT TRUCK UPRIGHTED & BRAKES CHECKED O-K. NOT WEARING SEAT BELT.