yep my speedo cable is mucho burnt as well. I even specified to the dodgey exhaust guy to make sure they do something about the speedo cable so it wont melt. They said yeah no problem, we'll secure it.
i picked the car up drive it home, lo and behold, half melted speedo cable. grrrr!
i just folded a small piece of zincalume and tack screwed it to the underbelly of the car.