There were four different tire/wheel combos: GT-1, GT-2, GT-3, and GT-4.
GT-1 and GT-2 were the narrow wheels front and rear, with "All-Purpose" tires on the GT-1 and "Traction" tires on the GT-2. Fronts were 4.8/4.00-8. Rears were 6-12.
GT-3 was the "High-Floatation" tire set, with the wider 16x6.50-8 (front) and 23x8.50-12 (rear).
GT-4 had narrow fronts (4.8/4.00-8) with "Studded" tires, and wider (23x8.50-12) rears with "Traction" tires.
GT-3 was not an option until 1964.
GT-4 was not an option until 1966.
Beginning in 1966, the tractor serial # indicated which tire package came as original equipment. I'm not sure how you'd know before 1966.
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