GMC Sierra Classic 2007 Owner's Manual page 444

Total Weight on Your Vehicle's Tires
Be sure your vehicle's tires are inflated to the
upper limit for cold tires. You'll find these numbers
on the Certification label at the rear edge of the
driver's door or see Loading Your Vehicle for more
information. Then be sure you don't go over the
GVW limit for your vehicle, or the GAWR, including
the weight of the trailer tongue. If you use a
weight distributing hitch, make sure you don't go
over the rear axle limit before you apply the weight
distribution spring bars.
Hitches
It's important to have the correct hitch equipment.
Crosswinds, large trucks going by and rough roads
are a few reasons why you'll need the right hitch.
444
Weight-Distributing Hitches and Weight
Carrying Hitches
A: Body to Ground Distance
B: Front of Vehicle
When using a weight-distributing hitch, the hitch
must be adjusted so the distance (A) remains
the same both before and after coupling the trailer
to the tow vehicle.