Sorry to dig up an old thread but I seem to have a very similar situation with vibration that comes and goes. I was hoping that you possibly had some resolution to this that you forgot to post an update.
My specs:
-21" Avante Garde wheels
-295/35/21 Conti DWS tires (front tires 25-30k miles / rear tires 6k miles) - One thing to note, I'm lowered and have spacers to negative camera in the rear shreds the inner portion of the tires in about 10k miles
-18-20mm wheel spacers of an unknown brand though they seem hub centric.

I can't seem to pinpoint the rear end vibration I'm having (it's barely in the steering wheel and I feel it in my ***). I've replaced my rear driveshaft (carden shaft iso blowout), and have replaced my rear wheel bearings with no avail. The vibration is never consistent. Most days it is there but some days it is very light. I can certainly feel it most around 40mph then it gets better up until about 65 and then as you get faster, the vibration gets worse. It is pretty bad when you get to 75.

Any other ideas on what to check would be great!

Thanks,
Andy

 

3 things to check

1. Do the lug bolts have the correct corresponding seat to the wheels - radius vs conical.
2. Do the wheels have the correct 71.6mm center bore that locates them on the wheel spacers
3. Are the spacers truly hub centric with a 71.6mm center bore tgat locks onto the wheel carrier and also with the proper 71.6mm lip to lock into the wheel?

What you describe sounds like a classic case of the wheel moving around at higher speeds due to any of the above.

 

That is a good point. It is something I need to look info, I never really measured the centers. The lug bolts as well aren't exactly as long as they probably should and I need to confirm the correct style of lug.

The spacers do seem to fit very snug on the wheels so I always assumed the center bore was correct on them. I will say they are definitely generic style, nothing extremely high quality like an ECS Tuning or H&R spacer for example.

I am considering getting new wheels here soon to eliminate the need for a rear spacer entirely.

One rear wheel also has a very small hairline fracture though it never leaks air at all. It is possible the wheel isn't perfectly round but I think I'd feel that constantly instead of sometimes the car drives smoothly. Does someone like Discount Tire have the ability to check for those sort of things?

Thanks for the help Petza!

 

With hub-centric wheels and spacers, the tolerance should be so tight that the only way to fully seat the mounting face of the spacer or the wheel to the rotor hub is by tightening the lug bolts (in a cross-pattern). If they slip right over each other, then that's not tight enough.

I also hope that you're not running stock lug bolts with an 18-20mm spacer, as that's not a safe condition where you'd only have a couple of threads engaged. In my opinion, any spacer of 15mm or larger should be the H&R DRM (or Adaptec Speedware equivalent) where they come with the special lug bolts that screw them into the wheel carrier and then you use your stock lug bolts through the wheels and into the threaded spacers. The bolt length needed to go through a 20mm spacer is too long and runs the risk of fracturing.

Let us know what you find.

 

The CTTS was never designed to run wheels larger than 21". My best guess is the OP has a poorly-balanced wheel and/or the tires have inner ply separation.