My stock setup was 7.5 fronts and 8.5 rears with tire widths of 225 and 245 respectively, and I just switched to 9 fronts and 10 rears with a set of new Continental DW's with 245 and 275 respectively. I've noticed the car feels slightly unstable at times on the freeway. It's almost as if the rear is slightly swaying back and forth. Do I need to get an alignment? I didn't think wheels and tires was included in the suspension geometry. I was also thinking that the wider wheels and tires would track road imperfections more. Or do I just need some time to break in the tires? Maybe a combination of all 3?

 

anytime you change your wheels you should get an alignment. Your contact patch has changed. Our cars have a natural negative camber in the rear (-1.0 degrees) and with the additional width of your tire, you are not getting the full contact patch of your tire when you are driving strait. Want proof? watch the inside of your rear tires. The negative camber is designed for handling...which comes into play when you are cornering and that is when you will feel the full effect of your wider tire...not in strait line like going down a highway.

one thing I would as is did you experience this after you bought your new tires? new tires are slick for bit when you first have them installed.

Another factor maybe the ruts in the road. Wider tires will travel in the ruts left to right. you may not have notices in your oem sizes (btw, they are 8.0f and 8.5 rears if you are speaking of the oem 19s)

get an alignment and then go for some test drives over some variety of roads, attempt to narrow it down to when it really happens (specific) or if it even happens at all anymore.

good luck!

 

You should check your tire pressure, first - it could be an easy fix. I had new rear tires put on (stock size) once and it was filled up to 40psi. Too much... only felt stiffer at low speeds, but at highway speeds it felt like I had no traction at the back. I keep it at 30 now. 35 is recommended.