I don't think there was ever a "Cobra R4-lug" rim.
So, a lot will depend on the backspacing of the rim.
I'm assuming it's a stock '85 GT/4-cyl setup up front (k-frame, spindles (87-93 are the same), etc)?
Also... why a 9" Cobra R 4-lug? I don't think that Ford sells a 4-lug rim with the official Ford Cobra R emblem.
So, then, why a 9" rim, and not an 8" rim???
Now, onto wider tire on Mustangs/cars...
It depends on the exact tire she wants to run. And, I do not mean just the size/width. It's pure BS that a 245/24/17 tire is "x wide" and "y tall". The same is true for any "size" tire. Check tirerack for tire specs.
It also depends a lot on the exact fenders and exactly on how they where put on and lined up. Many people with 4-eyes that "brag" about their big front tires will either trim the fenders, or push out/extend the bottom bar on the bottom on the fender (just in front of the tire). They also use spacers, rack limiters, and you rarely see then say the exact brand/model/year of the tire and the exact air pressure in the tires, if they have the lip pushed in, the exact height of the fenders with the car on level ground, and so on. And, yes, the year tire does make a difference since tire models change over the years.
So, be very very very very very careful about people telling you that you can put 335's on the front of a 100% stock 4-eye and it rides fine!
Also, is the car lowered? From '85, the lower isolators are likely gone. So, that will drop the front by very roughly 1/10" -> 1/4".
I've seen cheap *ss POS 245/45/17 tires that are as thin as a stock 225. So, again, the exact tires make a big difference.
Wider tires can be a problem with rubbing on the fender over bumps, or if the tires are turned and the car goes over bumps.
For example, when I got a new set of 17" Blizzaks snows, they were a tiny bit wider and taller than the same exact model and size that I got ~4 years earlier. So, the new tires had to go on the back, and I had to put the slightly worn back Blizzaks on the front. (I like to be able to drive my Stang about once every two weeks in the winter if the roads are nice and the weather is good. )
Last, here's my car: Mustang 86 - Mach1 Springs
1986 Mustang, sn95 5-lug setup, Cobra M2300K brake kit, ABS,
Mach1 Springs, Steeda 1/4" front spring spacers (placed on top of the isolator),
The front fenders sit ~26-7/8" from the ground and that's with 245/45/17 RE730s (@32 PSI) which are the correct diameter for the car.
The Mach1 springs are rated at 600lbs/in vs the 425/530 lbs/in stock GT springs.
ok, i know what wheel your talking about as far as the ford 4 lug cobra r and they do make these now. Ford Racing Performance Parts - [Part Details]
if you take a look closer these are made in a 17x8 wheel not the normal 17x9s that are the 5 lugs. heres a pic of my 84 with 96 gt wheels which are also 17x8s. you should be able to run these with a 245/45/17 on the front and with the rear quad shocks flipped be able to put a 275/40/17 on the rear. alot of people will argue the 275s on a 8" wheel but i have done this on numerous cars ive had and had no problems. sorry the quick anwser is yes,they will work.
you can run as wide as a 10.5 wide on the rear if you wanted(with out the quad shocks) the fronts will be good with the 17x8s. the front end you have now is the same as the 87-93 4 cyl fronts. the 5.0s from those years have the 11" rotors as opposed to the 10" that you have. youll be fine with 17x8s on the fronts and 17x9s on the rears.