Will 275/40s fit on 17x8 cobra rims? if not whats the biggest size i can put on there

 

They will. I have 275/40's on my 17x8 bullitts. Don't go any bigger than 275 on a 17x8, ever.

 

the general rule of thumb is no wider than 1" over wheel width, otherwise the wheel will "float" back and forth if you want to calculate tire width take the width in millimeters such as 275 and multiply it by .03936 that will give you the width in inches.