GMC 2000 Sierra - Manual (Page 247)

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Front Axle Reserve Capacity
This is the difference between your front GAWR and the
front axle weight of your truck with full fuel and
passengers. It is the amount of weight you can add to
your front axle before reaching your front GAWR.
The front axle reserve
capacity for your vehicle
can be found in the lower
right corner of the
tire/certification label, as
shown in this example.
In order to calculate the amount of weight any front
accessory is adding to the front axle, use the
following formula.
(W x (A+W.B.)) / W.B.= Weight the accessory is adding
to the front axle
Where: W= Weight of added accessory
A= Distance that the accessory is
in front of the front axle
W.B.= Vehicle Wheelbase