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Out here in VA, most of the tire size classes run like this:
A = 33" and under
B = 36" and under
C = 39.5" and under (though some are 40" and under)
D = 44" and under
These are usually the stock appearing vehicles, no tube chassis vehicles, no power adders (i.e. NOS, turbos (unless they came stock on the vehicle), or blowers), oh, and all are DOT approved tires
X = paddles, cuts
XX = tractor tires, anything goes
These are usually the big boys that run tube chassis, and can have the power adders!