1996, F-350, CREW CAB, 4X4, WITH 35,000 ORIGINAL MILES WAS TOWING 5TH WHEEL TRAVEL TRAILER IN MOUNTAINOUS AREA. EXHAUST PIPES CAUSED THE SPARE TIRE TO CATCH ON FIRE. THE SPARE TIRE WAS NEW AND HAD NEVER BEEN REMOVED SINCE VEHICLE WAS PURCHASED IN 1996 USED, WITH 3,000 MILES ON IT. THE SPARE TIRE WAS ABOUT 3-4 INCHES AWAY FROM THE EXHAUST PIPE HEAT SHIELD. ( NOT TOUCHING). REAR GAS TANK WAS NOT HARMED DUE TO QUICK ACTION ON FIRE EXTINGUISHING. DRIVER WAS UNAWARE OF FIRE UNTIL NOTIFIED BY PERSONS OUTSIDE THE VEHICLE. SPARE TIRE BURNED CLEAR TO THE CORE PRIOR TO BEING REMOVED AFTER FLAMES WERE EXTINGUISHED. *NLM