I found this link:
http://www.nhtsa.dot.gov/staticfiles/DOT/N...s/TP-139-02.pdf
I didn't find anything about a minimum acceptable treadwear rating, but I did notice new or revised test procedures for "endurance and low pressure performance" that seem to require the tires to hold up to at least 24 hours of 100% loading at 26psi plus 90 minutes of loading at low pressure or 20psi. See pages 44-49. Perhaps that's the part that the R-compound tires might have trouble with. Looks to me like it takes effect Sept. 1 of this year (for marking?) or Sept. 1 of 2007 (for performance?).
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