So I just got my MS6, and i need new tires for the stock wheels, so I'm just thinking about buying a new set of wheels and tires, plus maybe a drop.
So what is "TOO WIDE" for the MS6?
18x9, 18x8.75, 18x8.5, or 18x8? And I know the offset is another variable too.
and what Mazda 6 springs work on the MS6? the V6 or 4 cycl?
There is not really a too wide...it all depends on the amount of work you are willing do do (fender roll, fender pull, camber, cutting of the plastic fender liners, etc). Basically the higher the offset, generally the less work you will have to do to get them to fit.
Some dude on 6club.com named kid5 is running 19x10's now IIRC, so all those sizes that you listed will fit, all with some fender rolling done. And again, offset is the most important factor here, more so than simply width.
As for springs, the speed6 is a boat compared to the v6 and especially the 4 banger. With this in mind, the speed has different spring rates for its weight, and neither the i4 or v6 springs are really rated for the gargantuan, heavy ass speed..but they do indeed fit and if you HAAAD to get a set of regular 6 springs for whatever reason I'd opt for the v6 hatch/wagon for the extra weight they are rated for.
With all that being said, there are specific springs that are made solely for the speed, by a couple of manufactures; auto exe, H&R, and I believe therpmstore.com use to make/carry their own springs that had the perfect spring rate according to everybody from what I've read...but those have since long been discontinued.
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