[Richbot]

Currently I am using 285/40-17 Conti square on 17x10 ET25 and have run 285/35-18 on the same dimension 18's before as well. Drop 5 in aspect ratio for two wheel sizes up and there you are, 285/30-19.

As usual the issue is tire height not width. A 285/30-18 is a very short tire at just under 25", and fits for days. A 285/35-18 works fine width wise, but rubs the back of the fender liner. due to diameter. The 19's should be a bit better because they're a bit shorter in overall diameter than the 17's, which are a full 26" and most of the 19's are 25.8-9" tall. CAmber won't help. There's tons of room to the fender, the issue is the fender liners. The Conti 285/30-19 is .3" wider than my 285/40-17's, and 0.3" shorter (same diameter as the stock 245/40-18's, actually). So you'll be fine I think.

Didn't you have tires on these wheels before? CAn't you just go out and look at them and see how they're fitting and see how much room there is, then compare specs? Or is the car already on a lift somewhere?

I think the rear might be tight. That's a bigger tire in every dimension and like you say, ET30 would be better, but you're pushing the envelope anyway so who's going to be able to tell you without you trying it out? Nobody, that's who. Do it, for science.