it's impossible to tell if you will have rubbing or not. i repeat you won't know until its done. this is because the suspension is put together a little bit different from car to car. a mm here and a mm there adds up huge. even 18s can be big sometimew with a lowered car, mine for example. M5 inner fenders may be needed if he offset is higher than it needs to be. you will have to roll the fenders if your running 20s. and it would be a good idea to "pull" them as well. 20s do fit though, with only slight modifications if they're sized correctly.
this is not an exact science.
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