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      07-15-2012, 05:40 PM   #1
Merlin93
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Question F30 wheel/tire sizing & offset calculations?

Hey folks,

While I'm waiting for delivery, I'm researching wheels & tires as I'm thinking of upgrading the stock RFT P225/45-18 tires on my 328i Sport to non-RFT P245/40-18's, since those seem about the widest tires that the F30 supports in a square configuration. (I also considered doing 245's in the front and 265/40-18's in the rear, though rotations are easier with the square config.)

I'm mainly upgrading since I want nicer looking rims anyway, and I think the 225's that come stock are way too skinny when looking from the rear. Dropping the RFT's should improve the ride handling and comfort as well. I plan to store the stock wheels/tires put them back on at the end of the lease.

According to tirerack's tire size calculator the 245's should be able to fit on 18x8 rims (though it looks like just barely) so to be safe I'm thinking of moving up to something slightly wider, like 8.5" or 9" (the 9's would also support 265's.) Don't know for sure if that is needed though... and if I stick with 18x8 wheels I assume I don't have to mess with the offset, though the tires are wider so maybe I would?

I know the stock 18x8 wheels have a 34mm offset. If I went with an 8.5" wheel, that would reduce my inner clearance by 6mm and extend the wheel by 6mm further due to the rim alone, which might actually be fine, but I don't know how much play we have on the inner clearance. Assuming I don't want to move into that by too much, I was thinking of aiming for an offset of 28mm would keep the inner clearance the same, but would push the tire out by 12mm (which should be fine as there seems to be plenty of room, and it looks more aggressive anyway.)

When I search online to see what others have done with their own projects, I see people using 8.5 or wider rims with larger offsets (up to ET 40) which in my case would keep the tire at the same outside distance but reduce inner clearance by 12mm, which seems counter-intuitive to me.

I'm still trying to learn how this all works, so I'm hoping someone here can help educate me, or at least make some suggestions. I tried asking something similar on the tires sub-forum, but I don't think many people view that forum since I didn't get any replies.

I figure I'd ask to see what folks here have done (or plan to do) and I would appreciate any advice you can offer. I've been looking at rims but until I know what width and offset I should be using, I can only just window-shop.

Thanks!
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