This weekend I put on the new 19" wheels I purchased recently. SSR GT-10 very nice wheel. The car also has unrevised Z springs..all else is stock.
Once I drove it, I was getting a pretty strong vibration in the steering wheel at highway speeds (60 and over) and it did not seem increase with speed. None of this existed in my 17" wheel/tire set so I am thinking that it was not the car itself. I took the car into a tire shop yesterday had a 4 wheel alignment and all 4 wheels balanced.
While it is slight, there still is a vibration. Strange part is is not constant..meaning sometimes at the same speed the vib is there...sometimes it isn't. The shop suggested that the tires may be out of round (almost new tires BTW) as the wheels were easily balanced.
I am at a loss as to what would be causing this. Appreciated anyone's insight.

Have your wheels been balanced by the machine? There should be some magnetic weights on the wheels. That could be the problem. Also you should look into getting an alignment.

Yes both a 4 wheel alignment and all 4 wheels/tires were balanced.

Are you using centering rings if wheel center bore is not the same as hub diameter? Centering rings make a world of difference.

most likely you need hub rings

No .. currently do not have any centering hub rings on it. Just did a quick google on it and sounds like you may have the answer I am looking for. I wasn't aware of it so a huge thanks to you and az1anbeethoven.
Now the question is where can I get proper set?

Thanks again guys. I called Tire Rack and have the proper size rings on their way to me now.
Now I need to get some pics up on the site with the new wheels.

you can get it at any tire store...you just have to make sure what the center bore is on the aftermarket wheel and our G35 centerbore is 66.1mm...most likely, your wheel centerbore would be a 72.6-74.1mm...it shouldnt cost you more than $20 bucks for the whole set of hub rings

Thanks again for all the info. I went with Tire Rack simply cause they sell the wheel and therefore knew the correct size I needed. Looking forward to getting them on ASAP.

^^no prob...that's what we are all here for!

If it DOESN'T work, i had a similar situation to yours. I had all 4 wheels balanced, re-balanced, aligned, etc. What the problem was finally traced to was called the Road Force (RFD) of the tire. It was explained to me as another way the wheel is balanced that not all machines take into consideration. If your RFD is high, it will give you a rougher ride. So they end up re-mounting the tire on a different location on the rim (spin the tire on the wheel and remount based on a guided location). This is supposed to lower the RFD of the tire and give you a better ride.

Once they were able to lower the RFD, no more vibration. Though its a rare problem, it isn't unheard of. So if the hub rings don't help, this is just another place you could consider when continuing trouble shooting.

Thanks for the additional idea. Fingers are crossed that the hub rings will do the trick but if they do not, this would be plan B.
Did some look up on the internet and it would appear that not just any wheel balancing machine can determine the RFD. Since I went to a very high end shop, I would have thought they would catch if there was any issues...oh well.

Oooo I wanna see pics

Make sure when they install the centering rings that they seat COMPLETELY into the hub opening on the wheel. If there is a retaining clip, it must also be recessed. When I bought my BBS wheels, I had an issue with Tirerack sending me the wrong ones that were too thick and were not flush with the wheel/rotor mating surface. The tire install tech didnt notice and it caused me some grief as the bolts came loose very quickly (10 miles).

Luckily, I noticed it right away and pulled over to re-tighten, but it could have been a disaster at highway speeds as one of them was basically backed out totally off the stud. When the proper rings are installed properly, you should not be able to feel any ridge between them and the inside of the wheel.

http://www.sportcompactcarweb.com/pr.../photo_07.html

Thanks for the heads up and the link. I am suppose to receive the rings either Thursday or Friday. I will update once I have the chance to install them.