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what does et stand for?
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I'm not sure what the acronym stands for, but it's the offset of the wheel. The stock offset on MS3s (and maybe MZ3s and the 6es too?) is 52.5mm. It's best to stay as close to this as possible. I'm personally using 45mm offset wheels, I'm not having any major issues (it's possible there's some minor issue that I'm not noticing though).
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i'm looking at picking up 17x7 - 5 lug, VIA 690kg, 114.3 rims and wondering if it will fit with 225/45r17 Dunlop Winter Sport 3D tires.
IMO your wording is a little vague. I don't know anything about the wheels or tires, but based on their sizes they'll probably work on an MS3, so long as the offset is close to stock.
If you're asking if the tire will fit onto the wheel, that pretty much depends on 2 things. The diameter and width of the rim. Obviously the diameter is correct: 17". For a 225 width tire you would probably be slightly better off with a 7.5" wide rim, but 7" would probably work fine.
I don't know how much shopping/research you've done, so sorry if you're already considered this... If you're buying a dedicated set of winter tires you may want to go with more narrow tires, maybe 205s. Narrower tires apparently work better in snow and ice (rain too I bet).
I'm not a nerd-scientist (just a nerd) but I bet the reason they work better is because they have a smaller "contact patch" (the portion of the tire that actually touches the ground). The same amount of weight put onto a smaller contact patch will put more weight per square inch of tire. The additional weight will allow the tire to cut through snow/ice/water to the actual pavement where traction is made.
