What is the WIDEST tire I can go on my rear wheel ( 20 x 9.0 J ) ?

Some people have actually put a 305 Nitto 555 on the rears...Almost every reputable tire shop in my area wouldn't go over 285 because if you do drive it hard, you have the possibility of rolling the tire off the shoulder of the wheel. Especially when cornering.

I used a 305 Nitto on my SL. On my SLR I’m using a 315 Nitto. Both installed by Discount Tire.

Quote from dangerdarrell

I used a 305 Nitto on my SL. On my SLR I’m using a 315 Nitto. Both installed by Discount Tire.

On a stock wheel?

The SLR comes stock wtih a 11" wide wheel so running a 305 or even a 325 would not be an issue. However on a SL with a 9" wide wheel from the factory, while a lot do it, you really should not go any wider than a 275 in my opinion.

Quote from BryanL

On a stock wheel?

Yes. And, as Kyle D said, my SLR has an 11" wheel. I may be changing to 18" wheels this spring. Slingrazor has shown how the smaller wheel with a taller sidewall gives much better traction with the supercharger. I have driven his SS and it hooks up much better with the taller sidewall. Works similar to drag tires is you keep the pressure down. It allows for flex and the tire can grab better.

When considering a larger tire on your stock rim, always check the tire manufacturer's website for the wheel widths they recommend for the tire you are considering.