GMC 2011 Sierra Owner's Manual page 420

Say, for example, you have a 318 kg
(700 lb) snow plow. The total weight
of all occupants and cargo inside
the cab should not exceed 135 kg
(300 lb). This means that you may
only be able to carry one passenger.
But, even this may be too much
if you have got other equipment
already adding to the weight of
the vehicle.
Here are some guidelines for safely
carrying a snow plow on the vehicle:
Make sure the weight on the
front and rear axles does not
exceed the axle rating for each.
For the front axle, if more cargo
or passengers must be carried,
appropriate counter ballast must
be installed rear of the rear axle.
Counter ballast must be properly
secured so it will not move
during driving.
Follow the snow
plow manufacturer's
recommendations regarding
rear ballast. Rear ballast may
be required to ensure a proper
front and rear weight distribution
ratio, even though the actual
weight at the front axle may be
less than the front axle rating.
The snow plow manufacturer
or installer can assist you in
determining the amount of rear
ballast required, to help make
sure the snowplow/vehicle
combination does not exceed
the GVW rating, the front and
rear axle ratings, and the front
and rear weight distribution ratio.
The total vehicle must not
exceed the GVW rating.
Front axle reserve capacity is the
difference between the Gross Axle
Weight Rating (GAWR) and the front
axle weight of the vehicle with full
fuel and passengers. Basically,
it is the amount of weight that can
be added to the front axle before
reaching the front GAWR.
The front axle reserve capacity
for the vehicle can be found
in the lower right corner of the
Certification/Tire label, as shown.