I've grooved a set of 38x12.50x15 Super Swamper TSL's for trail riding and rock crawling and it really made those tires work better. It really loosened up the carcass and let the tires flex better when aired down. I don't remember the brand of grooving iron I used, a buddy of mine had a $70 groover and it worked fine, took about an hour per tire to get the pattern I wanted.

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Black, '07 Tundra DC, SR5, 5.7, 4x4, tow pkg