Does anyone know what is the largest width tire that I can put on my STOCK 18 x 8.5 inch rims without rubbing against any suspension components?

The stock tire is 255/55-18. I am thinking of going 285/50 or maybe even 305/45. But I am worried about rubbing.

Anyone done this before? Thanks in advance...

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2004 Porsche 911 C4S X51, Seal Grey / Black
2006 Porsche Boxster S, Guards Red / Sand Beige
2008 BMW X5 4.8i, Black Sapphire / Saddle Brown
2001 Audi A4 1.8tq, Laser Red / Black, APR Stage 2, H&R Coilovers
2002 Suzuki GSX600 Katana, Yellow