Handling problem
 

Need some advise. ok, my 08gt has 20x9's all around with 40 being the tire rim height. every time i'm traveling on the roads @ either 20mph or 65mph and going on a cornering turn or just rolling over slightly bumpy/washboard type of pavement my ass end will kinda shoot out like, what feels about few inches, and going on straight roadway it will,,well, feel like a minny fishtailing. On a good smooth roadway it feels GREAT. If i have to i will put some 18x9's on my car with maybe 45 tire height, would that help, because dam, it dam sure dont feel like a "sports car" when i roll over those imperfections on the highways and byways. Or do i need softer shocks/springs:(

partly I'm betting rubber bushings and a fairly tall ride height. Possibly a broken part but without more info it's hard to tell. I need to know a hell of a lot more about your car before I could start getting too specific but here's some things to think about:

First I'd start looking at your panhard bard and your upper control arm for replacement with something that comes with much more substantial bushings (poly or spherical where appropriate). After that lower control arms with poly bushings in front and spherical in back. Panhard bar adjustment is important. I'd need to drive your car to be too much more help.

Depends how bad the bumps are. If they're bad, that's partially how solid axle rears are. As one side gets pushed up, it causes a slight bumpsteer in the rear. Remember, our cars aren't sports cars. They're muscle cars with good handling. However, a properly adjusted panhard can reduce this effect. However, if you want sporty, keep in mind 20's are way too big and have a ton of rolling resistance. For wheel size, I suggest going only big enough to clear your calipers. If the aspect ratio is too high, lower your tire diameter to compensate. I still don't understand Ford's reasoning for larger tire diameters on S197s. Though a larger aspect ratio won't help your problem.