GMC Sierra Denali 2009 Owner's Manual page 324

You may go further and think the tongue weight should be
limited to less than 1,000 lbs (454 kg) to avoid exceeding
GVWR. But the effect on the rear axle must still be
considered. Because the rear axle now weighs 3,100 lbs
(1 406 kg), 900 lbs (408 kg) can be put on the rear axle
without exceeding RGAWR. The effect of tongue weight
is about 1.5 times the actual weight. Dividing the 900 lbs
(408 kg) by 1.5 leaves only 600 lbs (272 kg) of tongue
weight that can be handled. Since tongue weight is
usually at least 10 percent of total loaded trailer weight,
expect that the largest trailer the vehicle can properly
handle is 6,000 lbs (2 721 kg).
It is important that the vehicle does not exceed any of
its ratings — GCWR, GVWR, RGAWR, Maximum Trailer
Rating or Tongue Weight. The only way to be sure it
is not exceeding any of these ratings is to weigh
the vehicle and trailer.
4-48
Total Weight on the Vehicle's Tires
Be sure the vehicle's tires are inflated to the upper limit
for cold tires. These numbers can be found on the
Certification label at the rear edge of the driver door or
see Loading Your Vehicle for more information.
Make sure not to go over the GVW limit for the vehicle,
or the GAWR, including the weight of the trailer
tongue. If using a weight distributing hitch, make sure
not to go over the rear axle limit before applying
the weight distribution spring bars.
Hitches
It is important to have the correct hitch equipment.
Crosswinds, large trucks going by and rough roads are
a few reasons why the right hitch is needed.