GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 368

The front axle reserve capacity for your vehicle can be
found in the lower right corner of the Certification/Tire
label, as shown.
United States
4-42
In order to calculate the amount of weight any front
accessory, such as a snow plow, is adding to the front
axle, use the following formula:
Canada
(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