Help with matching tire size

[DTMdan]

So I started to consider finding a set/pair of E38/39 version Style 69s. That version is 18x8" front / 18x9.5" rear with low offsets. I'd like to keep two of my stock 18x8.5" X5 69s for the front and find two 9.5" rears.

Why bother? Wider. Concave. I would also be able to drop my 25mm spacer in the back because the 9.5" is et25 and that'd still give me 11mm more poke than I have currently. Since I can't afford 20"s at the moment I think this would be a nice upgrade and it would be pretty unique. And if I get LeMans flares eventually, I could put the 25mm spacers on the 9.5"s, which would be 17mm shy of OEM 10.5" rears... so that plan plus lowering springs would be pretty sick IMO and it wouldn't lessen the performance/handling as much as 20"s would (for a 3.0i).

Anyway my question is if I do pick up a pair of 9.5"s what size tire would I need so that it would match the 8.5" 255/55 up front? 275 55 18? 285 50 18? I'm not sure how to figure it out.

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Here's a couple pics of the difference between the X5 and the E38/39 versions...