The offset for the rims that I am looking to get are 45 mm.

 

Same as mine.

 

I have 18 x 9.5 on back with 285/40/18. I would not go smaller than 275.

 

I've got 18x10 in the rear with 285/35-18. My rear is dropped 1". No problems. If they fit a 10" I'd say you could go with a 285 on a 9.5" no problem. Just don't go with that width on the front. Try to match your tire diameters though. If you go with a narrower front like a 245, stick to a 40 or 45 profile. A lot of it depends on what look you're after though.

 

Note: For vehicles with 16" wheels from the factory:
-installing 17" x 8" wheels, use steering stop 4R3Z-3932-BA
-Installing 18" x 8.5" or 18" x 9.5" wheels, use steering stop 6R3Z-3932-CA

 

I saw those specs before and was under the impression that they were for the larger OEM wheels not the aftermarket. Has anyone here gone from the 16" wheels to an larger size without those steering stops?

 

This will tell you a little about offsets and backspacing.
I have 18x8.5" 50mm offset with 255/45-18s.
I have no steering stops like an OEM PonyPackage
with the 17s on it from the factory would have.
They rub at full lock.
I avoid full lock turns.
Evidently the shop didnt.

 

jd57 - I don't have personal experience with a 34mm offset wheel, but I'd be very nervous about using them with your setup. I think to get those wheels under the car where they should be, you need a 50+mm offset - Check with Discounttire on line, or tirerack, they're real helpful.

 

275/40/18 all around.

 

do 16 wheels fit on the gt's? i thought 17 was the lowest. im just asking cuz my uncle is giving me a set of 16 wheels with snow tires for the winter form his old mustang that he sold and im not sure if they will fit although he says they will.