The 305/30 is shorter than the stock rear tire so your speedo will be off a few MPH. I had that size on my car for a few thousand miles before doing the cam and they corrected the speedo in the tune. Now I'm running a 285/35 in the rear because I didn't like how short the sidewall was on the 305/30. That's crazy someone managed to stuff a 305 on a 9.5" wide wheel, I thought I had it stuffed on there on my 10" wide wheels.
Side note, how much did you pay for them if you don't mind? I've been looking for a set of these for a while and when they pop up they want stupid money for them unless they're in trash condition. I've been going back and forth and about these wheels and the Forgestar F14 since both will let me keep using the tires I already have. The F14 can be had in more aggressive offsets so I'm leaning towards those just because of that and they'll probably be a tad cheaper than all the CTS-V wheels I've been finding.
Thank you for this information. This is why I posted 1. to see pictures of anybody running this same setup 2. will there be an issues at all.
I've also been on the cts-v owners forum looking around. Some sizes will throw off the speedo as well as TC (traction control)..depending on how far away from stock specifications TC will kick in earlier or later based on the rotation of the wheels
Ultimately, I want to go with a forged wheel with custom offsets similar to 15SuperSport runs on his - but the price tag wont let me be great. Plus I wanted to do some performance mods (which my car goes in the shop this monday for those).
***also note i'm against lowering the vehicle for now and I do not want to look like I have a lift kit on the car. That is why I like 15SuperSport setup. He is at stock height but his rim/wheel combo fills in nicer than most