This calculator (go to the 'gravity box' one)
http://www.abe.msstate.edu/Tools/Volume/index.php
will give you the volume of your wagon. If you enter it as feet, you will get cubic feet. It is pretty easy to convert cubic feet to bu.
The better your measurements are, the better the results will be.
As well, corn can range from low 40's to mid 60's in test weight. A bushel is a volume mesurement, but we really end up buying & selling corn based on weight & devided by 56(lbs) and then adjusted by moisture content to 14.5% to give us a 'perfect' bu of corn.
Likely the raw corn you buy will be a little more or less than 14.5% moisture, and have a test weight of more or less than 56lbs/bu exactly.
Early in thefall I am lucky to get 190 bu of corn on my gravity boxes, when they are real high in moisture & the test weight is n the lower 50s. Later in the fall if the corn is dry and aged, I might get 235 bu on the same wagon, looks just as full. But the corn is down to 14% moisture, and the test weight is 61lbs/bu. My wagon is rated at 220 bu.
If that is all very confusing: Bushels is a volume measurement, but different quality level can make the same weight of corn take up more or less actual space. It is best to weight the loas, and test the corn for 'test weight' and 'moisture' to get a true value of the pile of corn.
--->Paul