I've done quite a bit of searching/researching but still can't quite nail done some 100% answers. I've got a 2011 GT with the base 18inch wheels. for those of you out there running 20inch wheels what offsets are you running? and how does it sit? the wheels i want are 20x9. with an offset of +38mm and like a backspacing of 6.5inches. i plan on running 255/35-20 tires on all four corners. of course i'd like to get as close to the fender as possible. but i'd rather tuck in a little than poke out. any thoughts? suggestions?

 

I think you'll have plenty of room. I ran this comparison....the tire on the left is yours, and the one on the right is what I presently have. With my set up I have plenty of room to go at least 1.5" wider and still have space to spare.

Your tire is only .4" wider. You even could go a 265, maybe wider without a problem based on this comparison.

Here's the link so you can play around and plug in different numbers:

http://www.rimsntires.com/specspro.jsp

 

I don't know how the backs pacing affects the offset but I have 35mm offset on mine and they still aren't flush. The ones that I see flush with the fender are typically under 30mm. The RTR are 33mm. 28 seems to do it though. I added 5mm spacers and it did the trick. FYI spacers subtract from the offset so now my wheels would be a 30mm offset.

 

Don't quote me on this but I don't think it would even come close to rubbing. I would do 275/35r20 and you should be fine easily. Trust me those 255/35s are going to look too skinny. I have 245/40r20 on mine with a 32 offset and it's not sticking out. It looks aggressive by still quite a bit before it would get to the fender.