Hello everyone - been gone fore awhile but I'm back.

What is the biggest wheel width/tire size combo I can put on the rear with an 18" wheel?

 

Biggest I would go is 18x10 with a 305/45/18.

But some have shoehorned in a 335, but then you have to start relocatiing suspension peices.

 

Do I need wheel spacers with 305s?

Also, what about a 315? I have 17" rims right now.

 

You would need a custom 17x10 to run a 315 and have it not rub on suspension stuff. Probably a 36-38mm offset.

I have heard of 305 fitting on 20's.

315 on an 18 or larger rim would need a 11 inch wide rim.

 

I've run 295/45/18's on 18x10's. It pretty tall at about 28.3" overall diameter.If you like a rake that is.My 07'Gt is lowered and with that set up you lose the level set up.Your speedo might go off a bit. I ran 255/45/28's on 18x9's up front. The ride was pretty good. 305/45/18's are 28.7" tall. Maybe just a bit tall and create more understeer.