First off 20's DO fit an e30. It has been done. You can't really do any suspension stuff however with a rim that big. I think he's doing the right thing staying with 16's as a larger rim really does slow you down. (wanna talk physics?
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I'm curious as to what type of suspension mods your gonna do? Thats alot of weight upfront. Also are you going with a different brake setup as well?
Off hand anyone know exactly what engine that is so I don't have to go look it up?