Originally Posted by mrpostman
I need some help! I am not 100% sure of all this technical stuff and I am totally lost with offsets. I have a 2012 Focus Ti with the 18" rims. Next winter, I am planning on getting 17" rims and winter tires, all with factory specs to ensure proper fit. If I was get a 215/50R17 winter tire and 17" rims with a 50mm offset, would that be acceptable? Why would anyone get a 40mm or 45mm offset when the factory 17" rim has a 50mm offset? Any help will be greatly appreciated!
With a lower offset wheel, the face of the wheel hub moves closer to the dead center line of the wheel. The more positive the offset, the farther past the center line of the wheel the wheel hub goes, so the wheels are, "pulled into," the car. The more negative the offset, the farther out the wheels are, "pushed." Many people will switch to lower offset wheels so as to achieve a wider track (either for visual effect or for perceived improvement in handling/grip). How far out the outside face of a wheel/tire combination actually goes depends on two factors, the width of the wheels/tires and the offset. If you want to keep the outside face in a similar place while switching to narrower wheels, you'd get lower positive offset to do so. So as an example, say your current wheels are +50mm offset, 18" diameter and 8" wide. If you switch to wheels that are 17" diameter by 7" wide, the wheels are 0.5" narrower to the center line from the outside and from the inside. 0.5" is approximately 12.7mm, so in order to achieve a similar visual appearance, you'd have to choose wheels with a +40 or +35 offset. Does that make sense?