Wider tires on sport rims?

spearfish25

As I wear my way through the stock rubber on my sport rims, I'm looking for my replacement tires. I've pretty much decided on the Ventus v12s. However I'd like to go wider on the tires, particularly the rear.

So, a couple questions:
1) How much wider can I go on the sport rims? Stock rear size is 275/35R19 and front 245/40R19. I don't want to push the limits or stretch/squeeze the tires.

2). How do I keep the rolling circumference the same? For example, if I can put 285s on the rear rims, do I still keep the 35 proportion?

Thanks in advance.

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2013 Cadillac V-Wagon, RIP Z