The flight instructor stated that prior to beginning the instructional flight, he visually confirmed there was fuel in each fuel tank. He did not use a fuel dip stick when he checked the fuel level and approximated the fuel level as 17 gallons. He said the flight would have required 8 gallons of fuel. About 10 miles from the destination airport, with the fuel selector positioned to the right main fuel tank, the engine began to run rough. He then selected the right auxiliary fuel tank, and the engine began to run smoothly again. About 4 miles from the destination airport and during the approach for landing, the engine began to run rough again. He then selected the left auxiliary fuel tank, but engine power was not restored. With the engine at idle power, he tried to restore engine power by cycling through the remaining fuel tanks but was unsuccessful. He then performed a forced landing to a field, during which the engine ceased to operate. The airplane sustained substantial damage to the fuselage. Postaccident examination of the airplane revealed no leaks with the fuel system and no useable fuel was found in the left and right main fuel tanks. The flight instructor stated that after the accident, he checked the left auxiliary/tip tank and there was 7 gallon of fuel present. The examination revealed no failures or malfunctions with the airframe and engine that would have precluded normal airplane operation.