I am 95% certain that the 18" wheels on S197 Mustang are 18 x 8.5", not 8" wide...That being said, I have seen as big as a 295 shoe-horned onto them for autox purposes...
As far as "how wide is too wide?" is concerned, it's really dependent on what aspect ratio you are to go with and how stiff the tire side-walls are...If you have a wide tire with a low aspect ratio (i.e. short side walls), they will be significantly stiffer and harder to mount on a rim...As far as street use goes, I wouldnt put anymore than a 275 on those stock rims...There is a direct trade-off between traction and tire wear...The "stickier" the tires, the softer the compound and therefore the shorter their lifespan...
That being said, the type of tire you go with will depend on what type of "traction" you are wanting? We need a little more information on what you seek...Do you want straight-line traction or grip for handling? Also, how aggressive do you want to do with traction for track vs. daily street use?
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'05 GT - Borla Catback, Koni Adjustable Shocks/Struts, H&R Race Springs, Roush Shifter, Hawk HP+ Brakes all around, Steeda Front Sway Bar & End-Links, Dunlop Direzza Z1 Star Specs on Enkei Race Wheels, Royal Purple Fluids
'07 F-150 Ext. Cab XLT Flareside - 5.4 Triton - Roush Dual Exit Catback
