juntsky, if you add width equally to both the inside and outside of the wheel then yes, you are correct, the offset will not change.

However, the OP mentioned that the width is added to the inside only which in turn changes the offset since the centerline of the wheel has now moved.

 

I believe there are some 17x10s for sale on s2ki in the wheel section. iForged wheels or something like that. Theyre supposed to fit with no modification at all to the fenders.

 

17x8.5 +58 17x10 +70 CE28Ns for sale https://www.s2ki.com/s2000/topic/103...#entry22728568

 

I'm about to get my rims wider. For best grip and handeling what is the best rim size to run a 275/40/17. 10 inch wide rim? 10.5 or 11 inch wide rim

 

For 275 I'd personally run 10.5. You don't want any slop, so a very, very mild stretch is preferable.

 

Agreed, 10.5" for a 275/40

 

Going to add 2" to the inside oem wheels? If you do this, you will want to add a 20-25mm spacer to regain some inner clearance.

 

I,m going to widen the stock front 17x7 to 17x 10.5 and use a 20mm spacer and run 275-40-17 nt05s. I will run the stock rear wheels in the front with the 5mm spacer with 255/40/17 nt05. Do you think the 20 mm spacer in the back will be good enough?

 

I really want to see this done. Sounds like a good setup.

 

With a 20mm spacer it would effectively make your rears a +70. That's a good fitment if your trying to stay clear of rolling your fenders. Aggressive camber in -3 range for track could have some inner rubbing issues if your lowered. If you have your fenders rolled, I would go with a 25mm spacer and make it a 10.5 +65. It will centralize the wheel better and give you plenty of leeway on either end for wider tire upgrades or alignment adj.