Chevrolet Forum : “Tire Size Comparision”

Assuming you stick with a 15 inch wheel, the closest size in diameter is 265/60R15, though 235/60R16 and 225/60R17 also fit, though you'll need new wheels.  Actually, you'll need new wheels regardless, cause you can't reasonably mount 265s on a wheel where 225s fit.  This is your friend: http://www.rims-n-tires.com/info_specs.jsp

Thank you for your answer, very appreciated.