I just got new tires and rims and have a friend telling I don't but I think I do. The old rims and ites are 2something/75/15 and my new ones are 255/55/18. On the Blazer below.

Not for a tire change. That would be rediculous if you needed an alignment whenever you got new tires and/or rims.

Good news is that the overall diameter of the255/55R18 tires is a match for your 235/75R15.

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Not unless your previous tires were changed because of alignment. I think inside tire wear is too much toe in....
If not, save your $ for something cool.

Thanks for the info.