C172 PLT MAKES AN OFF FIELD LNDG.

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I FILED 2 FLT PLANS AND CHKED WX; NOTAMS; WIND. NO ALT FILED AT THIS TIME AS WX OK. DURING FIRST FUEL STOP RECHKED WX RADAR SHOWED RAIN AT GTR. GTR +3 MI -RW HIGH OVCST LATER THAT DAY TSTM IN FORECAST. RECHK AND DEPARTED DHN FOR UBS. CLBED SBOUND TILL 3000 FT THAN ATC GAVE NBOUND AS FILED. TALKED TO ATC AND FSS ENRTE WX GTR +3 MI 1000 FT SCATTERED; 800 FT OVCST ENRTE ADDED GTR AS ALT. ATC ADVISORY WX MOVING NE AT 30 KTS FROM 25 MI NE OF MERIDIAN 30 MI FROM UBS TOOK WX DEV (IN LIGHT RAIN LIGHT CHOP). PASSING OVER UBS DID NOT EXECUTE VOR APCH AS HVY RAIN AND TURB WENT TO VOR AND ASKED FOR ILS RWY 18 AT GTR ATC COMPLIED. DURING APCH TWR ADVISED ARPT BELOW MINIMUMS. COMPLIED WITH MISSED APCH IN HVY RAIN AND TURB. I ADVISED MGM NEW DEST ATC GAVE ME HDG OF 150 DEGS BECAUSE OF TFC WX HVY RAIN AND TURB TOOK VECTOR S SE. LATER I RECHANGED DEST TO SEM APPROX 30 MI N OF SEM; HVY RAIN. SO MUCH THAT IT POURED INTO VENTS IN ACFT HVY TURB. ENG STARTED SPUTTERING SEVERAL TIMES (LIKE IT WAS RUNNING ON 1 CYLINDER). I STARTED DSNDING FROM 4000 FT TO MAINTAIN AIRSPD. ADVISED ATC AND ASKED FOR NEAREST SUITABLE ARPT. MY GPS SAID AO8. ATC SAID SAME AO8. I PROCEEDED DIRECT; USED SQUELCH ON RADIO; TO LISTEN AS I GOT LOWER. CAME OUT OF CLOUDS APPROX 2500 FT; SAW FOREST THEN GRAVEL ROAD. I LANDED ON GRAVEL ROAD. NO INJURIES; NO DAMAGE. ACFT HAD 7.2 GALS OF FUEL REMAINING. LATER I FLEW (AFTER INSPECTING) TO AO8.