The weight requirement for the tax deduction should have been another requirement.
It was meant to give a deduction for true equipment and capital investment (like tractors),
but has been applied to other vehicles under this lazy standard of looking only at the weight.
The 6,000 lb curb weight is the government distinction between “Heavy Duty” vehicles intended for more than passenger transport. Farm and other business often needs that capacity found in F-250 and larger type trucks. Weight limit is used for EPA requirements like the CAFE standards.
http://www.epa.gov/otaq/imports/glossary.htm#hdvhdt
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The Tax deduction now includes language to put larger SUV and crossovers into a more limited deduction.
http://www.section179.org/section_179_vehicle_deductions.html