If you burnt enough fuel unloading to make any difference either 1) you had a tremendous number of bushels, 2) you have the most inefficient tractor engine in the world, or 3) your cart has about a 4" auger on it. 

It would take that tractor 10 hours more or less at full load to burn 250 gallons of fuel, probably three times that at high idle with no load.  Did you spend  30 hours unloading 1500-3000 bushels of corn?