Telling how much tread is left - Drag Radials

I have run those a few times , when the tread is gone there isnt much left before the casing ....i ve gotten about 3 k out of a set. when the tread is worn down they seem to lose traction also.....personally I wont buy another set. if you do run them keep the air pressure up, at least above 20 lbs....i found 24-26 was about the best,any thing less and the centers cupped ..... we have found the taller ( bigger sidewall ) tires work better....good luck