I own a 2010 GT500 with the stock 19 inch wheels. Would like to know what the max tire size for the rear wheels would be. Currently using 285, Nittos.
wondering if 305s would fit? has anyone tried going that wide?

 

Yes a 305/35/19 will physically fit onto those 19x9.5 rear wheels, but it's not recommended.
The sidewall bulge will make it so you won't really get a 305 contact patch.
You're better off running a 285 with a better compound. It will fit more squarely onto that wheel, give a better contact patch, and stick better due to the better compound.

 

For a 9.5 inch rim the suggested max is 275. I know they run the 285's on them so you could get away with a 285. Sound like that is what you are running... I probably would not go any wider than that. So 285/35R19 is you best bet unless you want to move to a wider rim.

 

I see, thanks for the info!