I'm in the market for new wheels and I was interested in 17x7 +40mm offset. They fit on a WRX but I was told by Discount Tires they will not fit my RS. Why is that? Wouldn't that fit my 2003 RS that came with 16x6.5 +55mm offset?

The stock 6.5" wide rim w/+55mm offset leaves 1.085"/27.55mm of the rim on the fender side and 5.415"/137.55mm in the strut side (measuring from where the hub/wheel meet).

A 7" wide rim w/+40mm offset leaves 1.925"/48.9mm of the rim on the fender side and 5.075"/128.9mm on the strut side.

So the 7" wheel itself would be 1/4" further away from the strut and 3/4" closer to the fender.

Does the RS and WRX have something different under the fender?

I have Eibach Pro Kits and KYB/AGX's that are for both RS and WRX for 2002-2003.

 

+40 is too low for Impreza sedans. And waaaay too low for wagons and older Imprezas. For new WRX/RS sedans, I recommend 17x7.5 or 17x8.5 with 50~45mm offset. For your RS, I would probably stick with 17x7.5 +48 as this is a very common size/offset combo and there are plenty of great rims available in that size.