The tires are sometimes the lowest rated part that determines the GAWR but not always. The axle housing/bearings, springs, wheels, and frame also come into play. The most important is tires and wheels because the axle it self is usually rated much higher than the rest. Also air bags will help the springs rating. Find out what axle you have and then it's easy to look its rating up on Google.
Here are a few different ones that are common.
Dana 30 2,770 lbs Dana Corp.
Dana 35 2,770 lbs Dana Corp.
Dana 44 3,500 lbs Dana Corp.
Dana 50 5,000 lbs Dana Corp.
Dana 60 6,500 lbs Dana Corp.
Dana S 60 7,000 lbs Dana Corp.
Dana 70 10,000 lbs Dana Corp.
Dana 80 12,000 lbs Dana Corp.
Dana S 110 14,706 lbs Dana Corp.
Ford 9-inch 3,600 lbs Ford Motor Company
Ford 8.8 axle 3,800 lbs Visteon
Sterling 10.5 9,750 lbs Visteon
10.5 14 Bolt 8,600 lbs American Axle
11.5 AAM 10,000 lbs American Axle
10.5 AAM 9,000 lbs American Axle
Saginaw 9.5 6,000 lbs American Axle
