I WAS EAST BOUND ON INTERSTATE 8 EAST OF ALPINE, CA PULLING A TRAILER UP A 6% GRADE. I NOTICED SMOKE FROM THE REAR OF THE TRUCK. I PULLED ONTO THE SHOULDER OF THE ROAD TO INVESTIGATE. THE SPARE TIRE HAD SHIFTED IN ITS MOUNT NEAR OR AGAINST THE TAIL PIPE. THE TIRE HAD IGNITED AND WAS IN FLAMES. IT IS MOUNTED AGAINST THE BOTTOM OF THE REAR FUEL TANK. I EXTINGUSHED THE FIRE, AND REMOVED THE TIRE FROM THE TRUCK. I HAVE BEEN INFORMED OF 3 OTHER INCIDENTS OF THIS OCCURRING ON OTHER FORD PICKUP TRUCKS SINCE THIS INCIDENT. IN ALL FOUR INCIDENTS, THE TRUCKS WERE PULLING TRIALER UP HILL. MY CONCERN IS THE POSSIBILITY OF THE BURNING SPARE TIRE IGNITING THE FUEL IN THE REAR FUEL TANK. I CAN PROVIDE THE VIN NUMBER LATER IF YOU STILL NEED IT. *AK