My old truck used to have a box 7' wide x 6' tall and 9' long. which equals 14 yards. Now, obivously it wasnt a perfect cube but when volume hit say 2/3 of this cube it was full due to angle on the back and other factors. A full load would then be about 9.5 cubic yards. The truck drove reall equivilent to when I have about 2.1-2.3 ton of rock in it. using 2.2 ton that makes 463 lbs a yard. Mostly cotton wood and willow. 500 lbs is a reall good guess.
There is a difference in what is chipped too. Green dense woes weigh more than dead would for obvious reasons. Not sure how much less, but it is noticably less. I would guess more about 375-425 lbs a yard.
Stump grindings usually heavier. Depends on dirt quantity and most have dirt.