Widebody flares - Measure for wheels
 

Hi!
I'm having GT Racing widebody flares installed on my 1990 C4 (originally narrow-body). I'll be ordering custom wheels, probably Kinesis Supercup. Once the car is back together, I will drive it to a friend who has some experience with wheel and tire fitments. I think I can get 9" front wheels and 12" rear wheels, and I'm considering 255/40/18 in the front and 335/30/18 in the rear. Overall diameter would be about 26", or almost 1" greater then stock, AFAIK. I guess I'll have less pull from the stop, and more high end speed. Also, I guess my speedo will be off - new readings will be lower than actual.
I have adjustable sway bars so I think I could tune the understeer out of my tire/wheel selection.
The brands I'm considering for the street are Eage F1 ($1588), Michelin Pilot Sport ($1346), PZero System ($1174) and PZero Rosso ($1168). I saw where someone said they got alot more miles out of the Pilot Sports. My experience with Pirellis is that they are loud. Any idea as to how the Pilots compare? Is there any justification for the F1's? (other than their generous sponsorship and supercar cache?) Any thoughts about Rosso vs System?
What about measuring for the widths? Is there anything that I can calculate in advance, based on sotck offset, for the inside measurements (hub in?)
Any advice from the tire Gurus?
I suppose I should have broken this into 3 posts. Oh well...
Thanks!

CJV -
Thanks for your reply...

Here are the figures for OD, per the Tire Rack site:
Eagle F1: f=26", r=25.9"
Michelin PS: f=25.9", r=26.1"
P0 Rosso: f=25.9" r=26.1"
P0 Assym: f=26.1", r=26.1"

So, they all seem to match up pretty well, if that's what you meant.

In 1990, all cars came with a psm-type system, I think called PDAS, which is always on.

Yes, not stated for brevity, as it was obvious based on tire sizes...
Thanks again for you interest!
Jack