PLT OF SMA AT CRUISE HAS ROUGH ENG; NOTES FUEL GAUGES DO NOT LOOK NORMAL. REQUESTS AND RECEIVES VECTORS TO NEAREST ARPT BUT UNABLE TO MAKE IT. MAKES AN EMER LNDG ON HWY.

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I DEPARTED ASH; NH; IN A C180 WHICH I OWN AND OPERATE; ON AN INST FLT PLAN FOR AOO; PA. ENRTE TIME FOR THE APPROX 330 NM LEG WAS COMPUTED TO BE 2 HRS 45 MINS (AT 130 KTS GND SPD) WITH 4 HRS FUEL ON BOARD. DURING A PREFLT INSPECTION BEFORE THE FLT; I VISUALLY INSPECTED EACH OF THE 2 WING FUEL TANKS; FELT AND SAW THE AMOUNT OF FUEL AND SECURED BOTH CAPS. A PAX; WHO IS ALSO A PLT AND FLT INSTRUCTOR; MR A; OBSERVED ME PERFORMING THE PREFLT INSPECTION AND HEARD ME VERBALLY CONFIRM THAT WE HAD FULL FUEL TANKS AND ADEQUATE OIL BEFORE DEP. THE FLT WAS UNEVENTFUL EXCEPT THAT I THINK I NOTICED A FAINT SMELL OF GASOLINE AFTER LEVELING OFF IN CRUISE FLT AT 6000 FT BUT THE ODOR DISAPPEARED ALMOST IMMEDIATELY. AFTER THE LEVELOFF; ABOUT 10 MINS INTO THE FLT; I SWITCHED FROM THE 'BOTH' POS ON THE FUEL SELECTOR TO 'L' TANK. I RAN ON THE L TANK FOR 1/2 HR AND SWITCHED TO THE 'R' TANK. AFTER ANOTHER 1/2 HR; I SWITCHED BACK TO 'L.' WE WERE NEARING THE AVP; PA; AREA; RUNNING ON THE L TANK; WHEN MR A OBSERVED THAT THE FUEL GAUGES IN THE WING ROOTS APPEARED 'DIFFERENT' FROM USUAL IN THAT THEY WEREN'T BOUNCING AROUND AS THEY USUALLY DID AND WERE BOTH INDICATING 3/4 FULL. HE SAID THEY APPEARED TO BE WORKING BECAUSE THE QUANTITIES WERE GRADUALLY LOWERING BUT THAT THE INDICATIONS WERE UNUSUALLY EVEN AND STABLE. I BECAME CONCERNED SINCE THIS IS NOT A NORMAL INDICATION. I CALLED AVP APCH AND TOLD THEM WE MAY HAVE A FUEL PROB AND NEEDED TO LAND AT THE NEAREST ARPT. ALMOST IMMEDIATELY; THE ENG ROUGHENED AND APPEARED TO PARTIALLY LOSE PWR. I RICHENED THE MIXTURE; SWITCHED TO 'BOTH' FUEL TANKS; AND APPLIED THE CARB HEAT. THE ENG SMOOTHED OUT AND AVP APCH CLRED US FROM 6000 FT TO 3900 FT. WE WERE BEING VECTORED FOR THE ILS RWY 4 APCH AT AVP; STILL IMC AT 3900 FT BUT BREAKING OUT OF THE CLOUD LAYER; WHEN THE CTLR CLRED US TO 3000 FT. WE WERE ABOUT 12 MI FROM THE ARPT WITH GND CONTACT WHEN THE ENG STARTED TO ROUGHEN; MISS; AND SPORADICALLY LOSE PWR AGAIN. I SAW THE HWY UNDER US AND MADE THE DECISION TO LAND ON IT INSTEAD OF TRYING TO REACH THE ARPT. I TOLD AVP APCH WHAT WE WERE GOING TO DO AND EFFECTED THE LNDG WITHOUT INCIDENT. TFC WAS LIGHT AND IT WAS NOT DIFFICULT TO LAND THE AIRPLANE IN A CLR AREA; AWAY FROM AND WITH THE FLOW OF TFC. AS SOON AS WE WERE STOPPED I SHUT DOWN THE ENG AND WE PUSHED THE AIRPLANE OFF THE ROAD AND CLR OF TFC WITH THE HELP OF PASSING MOTORISTS. I HAD BEEN ABLE TO CONTACT AVP APCH ON 121.5 MHZ AND ADVISE THEM THAT WE WERE ON THE GND WITH NO INJURIES OR DAMAGE. AS WE WAITED FOR HELP; I CLBED UP ON THE WING STRUT AND SAW THE L FUEL CAP HANGING BY THE CHAIN. I WAS SURPRISED BECAUSE I REMEMBERED CHKING THE TANKS FOR QUANTITY AND SECURITY ON THE PREFLT. THERE WAS LITTLE OR NO FUEL IN THE L TANK AND 2-3 INCHES IN THE R. FROM THE CALIBRATED FUEL STICK I USED TO DIP IT; IT APPEARED TO CONTAIN ABOUT 5 GALLONS. A PASSING MOTORIST WHO WAS ASSISTING ALLOWED ME TO USE A CELLULAR PHONE AND I CALLED MY MECH IN CINCINNATI. HE IS KNOWLEDGEABLE ABOUT THIS AIRPLANE AND ITS SYS AND; WHEN I DESCRIBED THE SIT; HE EXPLAINED THAT IF THE CAP HAD COME OFF EARLY IN THE FLT WHEN THE TANKS WERE BOTH FULL; SUCTION WOULD OCCUR NOT ONLY IN THE AFFECTED TANK BUT THROUGH THE VENT LINE THAT CONNECTS THE 2 TANKS AT THE TOP. FUEL COULD BE SUCTIONED OUT OF BOTH SIDES. HE AGREED THAT THAT WAS WHAT APPEARED TO HAVE HAPPENED. THE STATE POLICE RESPONDED AND; WHEN I DESCRIBED THE PROB; ALLOWED ME TO FILL THE TANKS WITH 15 GALLONS OF AVIATION FUEL WHICH WAS SECURED FROM THE ARPT. I RAN THE ENG; CHKED ALL THE SYS; AND THEN SHUT IT DOWN FOR AN OUTSIDE CHK WHICH WAS SATISFACTORY. THE POLICE CLOSED THE HWY FOR APPROX 5 MINS AND I TOOK OFF ALONE IN THE AIRPLANE AND FLEW TO AVP WHERE MY PAX MET ME AFTER ARRIVING BY GND TRANSPORTATION. FBO AT AVP FILLED THE TANKS WITH 43.7 GALLONS OF FUEL AND; AFTER ANOTHER TELEPHONE CONVERSATION WITH THE MECH AND A THOROUGH CHK; WE TOOK OFF FOR WASHINGTON; PA; ABOUT A 2 HR FLT. ON LNDG AT WASHINGTON; WE REFUELED WITH 25.9 GALLONS AND PROCEEDED ON TOCINCINNATI; AGAIN WITH NORMAL FUEL CONSUMPTION. I BELIEVE THE L FUEL CAP CAME OFF INFLT AFTER DEP FROM NASHUA; NH; AND THAT; BECAUSE THE TANKS WERE FULL; THE SUCTION CREATED BY THE MISSING CAP CAUSED FUEL TO SIPHON OUT OF BOTH TANKS THROUGH THE VENT OR XOVER LINE. I AM CERTAIN I REPLACED THE L FUEL CAP ON THE PREFLT INSPECTION AT NASHUA; BUT IT IS POSSIBLE IT WAS NOT SEATED PROPERLY. THE CAP IS A MONARCH TYPE WITH THREADS AND IT COULD HAVE BEEN POSITIONED IN A WAY THAT LOOKED CORRECT BUT WAS NOT SECURE AND CAME OFF INFLT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THE FAA HAS REVIEWED THE INCIDENT AND INDICATES THAT NO FOLLOW UP ACTION WILL OCCUR. THEY APPARENTLY FEEL THAT THE PREFLT WAS DONE CORRECTLY BUT THE CAP ONLY APPEARED TO BE TIGHT AND WAS NOT. RPTR HAS COMMUNICATED WITH CESSNA TO DETERMINE IT IF IS POSSIBLE TO SIPHON FROM BOTH TANKS WHEN SELECTOR IS ON AN INDIVIDUAL TANK. APPARENTLY IT IS POSSIBLE.