I have thought about getting a new pair, perhaps an 18x8 or 18x8.5, I want to get something around 18-21 lbs. I have a set of wheels from my old car sitting in the garage, I don't really want to have another set. I am not so much in love with the factory rims. I do want to stay with 18s. I guess I could get another set of the rears. What would be the issue with doing that on my car?

 

No issues running a square setup w/ all 4 rear wheels. Here's a good read from tirerack~
Staggered Tires: Why or Why Not - Ben's Blog | Tire Rack

My Wedssport SA-10Rs 18x9.5s weigh 20 lbs each and don't cost a mint, they have a couple other styles in the same price range, might give them a look~
WedsSport Wheels

 

The stock sport wheels (rear) have an offset of 45mm and can be found on EBay remanufactured for just over $200 each. If you can essentially double your tread life losing the staggered setup, then this mod pays for itself the first set of $200ish tires you purchase. I'm still confused as to why nobody has done this? A 245 might be a bit too wide for the front. I'm thinking you could split the difference and run 235s all around. The difference in the rear would barely be noticeable. Add 10mm spacers all around and I bet this setup would look good.

 

That's what I did on our first G35 with the staggered split spoke 18s, the 235s worked out pretty well TBH.

 

That's right. There were two versions of those split spokes. One version was staggered and the other that was later available through the performance tire and wheel package consisted of 4 wheels with the same width all around. I'm not sure if either had the clearance needed to clear the sport brakes as I wanted to replace the Duckfeet wheels when I bought my car with these wheels and it was not an option. 235/50s all around with 4 of the 8 inch Duckfeet rims and 10mm spacers is probably the sweet spot.

Does anyone know if outfits like Discount Tire and Costco will still offer the free rotation service if you use spacers?