Op, come to the SA AFD on Sunday, I'll be at the Adelaide E Series setup with my NC. I'll gladly run through how to fit 10" wide rears, and even show you it done.

So a 10" rim WILL fit on an early E series? Legalities, aside?

EB GT's came with 17's from the factory so you can legally run 19's as long as the rolling diameter is within tolerances like others have said here. I also have to agree with the previous suggestions on width, I am running 245 19's on my EB and don't feel you would get any further performance gains from going any wider. Also have to remember that a big and wide low profile tyre will not do too much for you unless you have given your suspension a pretty good work over (lowered hardened springs with god shocks, good quality bushes, ball joints, etc.)

And again, props for doing some research yourself!!