Which spacer you get is dependent on what your lug spacing is (and also what lug spacing you want to end up with.) For example.. If my Jeep's lug spacing is 5x4.5 I would be getting a 5x4.5 spacer. You can also get spacers that change lug pattern. I.E 5x4.5 to 5x5, etc. They go by a different name, but I can't think of them at the moment.
So say... If your axle uses a 5x4.5 lug pattern, but your wheels were 5x5, you could use one of those adapter-spacers to fit those wheels that would have ordinarily not fit.
There's a lot of people who say spacers are dangerous, and lots of people who say they are fine. I've never experienced them myself, but I would suggest when installing them that you follow all of the procedures exactly to spec. Good luck, and safety first.
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