I run Nittto 555 Extreme 295-35-18. my car is lowered though
http://i.imgur.com/1EnuMWq.jpg I couldn't run a wider tire if i tried!
If you are worried about grip, yea there are benefits to running an extra inch of grip on the tire but the cost / performance ratio is really nill these days.
The technology in a tire today is grippier that the super wides of the 60s/70s.
The tire size 285/295 for our Mustangs is what is optimal for the most grip on an 18" tire. The 17" tire gets more grip because there is room for sidewall flex, that is why 17-45-305's exist.
in the basic sense.... 12" of tire contact patch allows side wall flex to give you almost an extra 1" of contact patch for a second, giving you 24" of contact patch to each tire, on the car when you launch, hence more grip. There is not enough rubber on the side wall of an 18" wheel to do that.
if you want more grip than what a 295-35-18 or smaller tire will give you, you can help grip by doing....
1) firming up the
lower control arms to plant the wheels to the pavement,
2) subframes to stop the car twisting when you launch.
3)
panhard bar to keep strait line stable and corners predictable
4) torque arm to increase rear grip even more! it also helps launching and wheel hop.
so you are ultimately paying for show when you get any tire larger than 295-35-18" wheel.
you could run 305-XX-18 tire, where XX is the sidewall height. The bigger the number the more sidewall flex you get. but then you have to roll the fenders, change the computer to take into account a taller tire mileage difference and be totally ok that your car will look kind of weird. You will also have to take into account your stopping power because of the unsprung weight.