20" Tire Upgrade-2005 Z71

[fxst]

I have a 2005 Z71. It has 20" 2008 LTZ wheels with the OEM 275-55-20 tires. How much bigger can I go if I install a leveling kit? I would like to go around a 305 or 315. I don't want to lift it as I do a lot of highway driving. The only off road is on the beach.
Thanks for any input.