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Do you think there will be much guard work fitting that offset though? I mentioned earlier front and rear are already as rolled as possible but I'm not overly confident/keen on doing more work.
Might be the **** pedders coilovers too.
There only the worst coilover on the market for these! I've seen so many blow out on this forum, the G8 board and in real life. My mates FG turbo got a few months outta his and barely drove it
I didn't do a thing besides rolling rear guards and grinding down the bumper tabs, didn't even relocate them.
setup in that photo is with 5mm spacer on front and 10mm on rear, so effective 8.5 +23 and 9.5 +18 with 245/35/19 falkens. You couldn't drive it like that daily in my opinion, drove horrible. Might be the **** pedders coilovers too.
9" 20P will fit the front with no guard work IMHO but seeing as yours are rolled you won't need any neg camber.
I think your biggest issue rear would be tyre size a 265 on a 10.5 with 27P offset would be better for guard clearence if your that low and don't want a stiff suspension.
The Vossen you mention will fit easily without much work. Lots of G8s don't need any bumper tab trimming unless there got wide tyre which hit it (like 295s)
Only reference picture close to those specs is a G8 on 10" 20P wheels all round, use willtheyfit.com to compare your new wheels to this guys fitment.
*Teaser* 20x10 +20's - Pontiac G8 Forum: G8 Forums - G8Board.com