Join Date: Jun 2005
Location: Jeddah, Saudi Arabia
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It's not the backspacing you should worry about. With rim width (6.5) being equal, From 55 to 42, you'll have 13mm of extra backspacing, so no worries there.
What you should worry about, however, is rubbing. Despite the fact that they will physically fit on the 6, the Fusion's wheels will stick out 13mm more than Mazda's OE steel wheels in relation to the wheel wells and fenders. And with the inherent sidewall "bulge" of the 205/60 tire size, you will run the risk of rubbing.
Another thing you should consider is center bore. I haven't made actual measurements, but from what I saw, when I considered buying a set of Milan (same as Fusion) wheels to use on my old 6, the center bore appeared to be significantly bigger than that of the 6 (67.1mm).
So it might not be that good of a deal after all. Should you actually go for the Fusion's wheels (with rubbing issues notwithstanding), you'll need to spend extra money on buying (or even fabricating) centering rings. And those are especially tricky to use with steel wheels.
Another issue. With such as "extreme" offset as that of the Fusion's wheels (as compared to the 6), there will be abnormal stress on the lower ball joints. To put it as simply as possible, road forces will load the ball joints a bit sideways, causing them to be stressed more to one side instead of being loaded equally. I once used wheels with a 40mm offset, and had to change the lower ball joints (entire control arms, actually) twice in 10,000 kilometers! Not good!
1994 Mercedes E320 sedan. How it's done right.
2009 JDM Mazda 6 2.5 MTX #1. Most people are mediocre by choice. I'm not most people.
2009 JDM Mazda 6 2.5 MTX #2. A fool and his money - but it is his money.
1991 Mitsubishi Lancer 1.5 GLX-i sedan. Mom's. Don't ask.