C172 PLT MADE AN OFF ARPT LNDG BECAUSE OF POSSIBLE FUEL QUANTITY PROBS.

Narrative

I WAS ON A VFR XCOUNTRY ON A ROUND ROBIN BACK TO MY BASE. I WAS FLYING A C172. WE HAD CALCULATED 3.5 HRS OF FUEL ONBOARD FOR A 100 MI LEG BACK INTO OUR BASE. 35 MI OUT FROM OUR BASE; I OBSERVED THE FUEL GAUGES ONE AT A TIME DROP OFF TO THE PEG. (THAT IS READING ZERO FUEL.) I IMMEDIATELY TOOK ACTION TO SEE IF WE WERE VENTING FUEL; BUT THAT PROVED UNSUCCESSFUL. I WAS IN CONTACT WITH A FLT SVC AND THEY VECTORED US TO THE NEAREST ARPT. THE ARPT WAS PRIVATELY OWNED; AND WAS COVERED WITH SNOW; AND WE WERE UNABLE TO LOCATE THE FIELD. THE ONLY OTHER ACTION WE COULD TAKE WAS TO LAND IN A FIELD (THAT IS; A SOFT FIELD LNDG). WE SUCCESSFULLY MADE A SOFT FIELD LNDG IN A FARMER'S FIELD. THERE WAS NO DAMAGE DONE TO THE AIRFRAME OR TO THE 2 PLTS ONBOARD. WHEN WE DID A VISUAL INSPECTION OF THE FUEL SIT; THERE WAS ADEQUATE FUEL ONBOARD. WE HAD NO WAY OF KNOWING THE REAL PROB UNLESS WE GOT THE AIRPLANE ON THE GND. I KNOW THAT YOU ARE SUPPOSED TO CALCULATE YOUR FUEL IN RTE BY THE CLOCK AND NOT THE GAUGES; BUT TO WATCH THE FUEL GAUGES DROP OFF ONE AT A TIME TO ZERO WAS ENOUGH FOR BOTH OF US TO DECIDE TO GET THE AIRPLANE ON THE GND SAFELY; SO AS TO CAUSE ZERO DAMAGE TO PEOPLE ON THE GND; THE AIRPLANE; OR THE 2 PLTS ONBOARD.