Basically, it's a marketing ploy. GM has two different 3/4 ton trucks...
one with 6 lug studs, the other with 8 lug studs. The 6 lug truck is rated
to 7,200 GVWR while the 8 lug truck is rated to 8,500 GVWR.
Same with the one ton trucks... one is rated up to 10,000 GVWR (the plain
one ton truck) and other above that rating based on how it was built.
Heavy duty is a relative term. The 400 series and up in Chevy, Ford, and
Chrysler are called medium duty. Even many "lighter" dump trucks are still
considered medium duty. Heavy duty doesn't really kick in until you get
into very large straight trucks or semi's.
GM has two different 3/4 ton trucks...
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