Gravel weighs about 2500 lbs per yd dry. If wet it's more like 3500 lbs/yd. Depends on the material but that is a good place to start. Dirt can be from 2000 dry to 2800 wet. These are averages only.
So to awnser your question "how much dirt is 6 or 7 tons", it's approx. 5-6 cu. yds. Also I driven a lot of 50's-60's truck loaded with that much and more and never had any problem stopping.
