FUEL EXHAUSTION FORCES PLT OF SMA EXPERIMENTAL ACFT TO PERFORM AN EMER OFF ARPT LNDG ON HWY.

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DEPARTED 22G WITH FULL FUEL FLYING TO C59. CREW MEMBER (PLT NAVIGATOR) IN REAR SEAT BECAME SICK ABOUT 35 MI FROM C59. I WAS NOT AWARE THAT THE AUTO FEATURE ON OUR LORAN ONLY CHANGED WAYPOINTS AUTOMATICALLY AND NOT (GRI) SO WE WERE STILL ON 9960 CHAIN. THE LORAN DIDN'T LOCATE THE ARPT SO WE CIRCLED TO LOOK FOR IT. UNSURE OF THE AMOUNT OF TIME WE CIRCLED; AND WANTING TO LAND BECAUSE OF SICK CREW MEMBER; I STARTED BACK TO A PRIVATE STRIP I SAW ON THE WAY; AND CALLED (KIRKPATRICK). WHEN ONE TANK WENT DRY; I CHANGED TANK AND RESTARTED. THEN STARTED THINKING I MIGHT NOT MAKE IT ANOTHER 12 OR 16 MI BACK; OR ACROSS A LAKE IN FRONT OF ME SO I CHANGED DIRECTION AGAIN BACK ON ORIGINAL COURSE WHEN THE OTHER TANK WENT DRY. I DIDN'T KNOW WHERE ELSE I COULD MAKE A SAFE LNDG AND SAW THE HWY. THERE DIDN'T SEEM TO BE MUCH TFC SO I LANDED ON RTE 67. AFTER TALKING WITH FAA AND LCL POLICE I WAS ABLE TO TKOF FROM HWY. OUR SMA HAS A STD PWR PLANT AND HOLDS A TOTAL OF 32 GALLONS OF FUEL; OR 3 1/2 HRS ENDURANCE. I PLAN FOR A FUEL BURN OF 9 GALLONS PER HR. I AM LOOKING INTO THE POSSIBLE SUCTION OF FUEL FROM TANK VENTS.