They use pretty Hefty wire on them now. It is something like .035" on the 1.5cc engines and they run 35' line to the tether
Rules:
http://amrca.com/rulebook/rulebook.doc
Class I 1.5cc .9mm (.035 in.) change at 286 km/h (178 MPH)
Class II 2.5cc 1.2mm (.047 in.) change at 306 km/h (190 MPH)
Class III 3.5cc 1.4mm (.055 in.) change at 312km/h (195 MPH)
Class IV 5cc 1.5mm (.059 in.) change at 310km/h (192 MPH)
Class V l0cc 2.0mm (.079 in.) change at 345km/h (214 MPH)
A Saito powered car 1988