Pork Pie has lots of stories about that vehicle - and photos, too. I 'spect he's posted them somewhere on here. Maybe there's even one or twoof the tractor in the lr.com gallery under his name. Take a look.
As I remember, the top speed was "classified" by JCB, but the thing was known to have gone somewhere in the lower 80s, I think it was, while on the salt. It was quite the rig to watch, I remember. I s'pose that if I were to dig out that old 'puter where I usta store photos -- I'd find it. That's a lot of work. See if you can find it easier.