I hired a tri-axle out of Connecticut twice to pick up my PA coal. 18 tons per trip....legally. If his body was a little larger he could of hauled more so I'm told. I still have the scale receipts somewhere, one from Calvin V Lenig the other from UAE Harmony.
I just called my trucker. He said the body on his truck was 28x7.5x6. That by my calculations would be 46.7 yards. Now I just have to fact check that against the average weight of a yard of coal time 48.7 and see if it is in the ball park of 18 tons. When I find the standard weight per yard I will update this post. My point being because his truck was sized for hauling gravel and stone his body is a bit smaller than a tri-axle that was sized right for coal which is lighter for it's volume.
EDIT: OK, These numbers where making no sense to me. Looked up tri-axles for sale and got body dimensions that are much different from what my trucker told me on the cell phone. He must of misunderstood my question and gave me dimensions for a tractor trailer dump body. Most of the tri-axle bodyies where around 17' which makes more sense 17x7.5x6 would be more like 28 yards times 1350 lbs per yard would equal 18.9 tons. Weight per yard can vary some but those figures are ball park for a full BODY.