I found some wheels on here that are on an 04 GT. Here are the sizes
Fronts: 265 35 Nexen N3000 Tires mounted on 8.5" wheel
Rears: 295 35 Nitto Invo Tires mounted on 9.5" wheel
would there be any problem with fitting on my 08 GT?

 

Whats the backspacing and or offset?

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I'm unsure right now.. here's the link to the guy selling the wheels https://mustangforums.com/m_5251488/mpage_1/key_/tm.htm

 

they should fit....just depends on the offset and if you are lowered a lot.....i currently have 20"x8.5" with +24 offset on my 2007 GT with 255/35/20 on all 4 corners with the stock suspension....i have no rubbing issues, just a slight very minor vibration for the nexen/lexani n3000 tires....i just cant afford to replace the tires yet....nittos are in my future

 

ya these wheels are 18's and idk the offset.. so i don't think i'll have the same issue since you have the 20's.. i just wanna know if it will look stupid or if it would be any trouble for it to fit because i don't wanna buy the wheels and then have a problem with them

 

When you get wider than 9" wheel and 275 tire with the 99-04 offset (24mm) they stick out past the fender and look bad on 05+.

 

I can just tell by the look of the mounting
hub that they will be farther out than OEM
05-08 rims.

 

so you don't think it will look good? i've heard people putting the 99-04 saleen style wheels on the 05+ and saying it sticks out past the fenderwall a little and that it makes it look more aggressive or something.. idk what to do cuz if i get a body kit i'm probably gonna need diff wheels and idk if i want 20s.. =/

 

It could look good but if they stick out, dont plan on getting her lowered.
You could start rubbing the upper fender well lip and start throwing rocks
and dirt all up the sides of her.
I see the spanish speaking people around here with the rims sticking so far
out that you can see ALL the tread. Not my look. Sticking out too far also
just looks like you could not find or afford the correct rims. If its a deal you
just cant pass up, buy em. If for the same amount of change and a little
more $$ thrown in you can get what you want and what fits and looks good,
then pass them by and wait for the good stuff, not something you dont know
will fit or look good or not rub.
Unless you can try before buy, I wouldnt go that route.
Its almost a 1" difference that they will be farther out.
Get a straight edge and take some measurments then sketch it out to see.

 

The front 8.5" are 157mm back space with a +35 mm offset and the 9.5" rears are a 170 mm back space with a +35 mm offset