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I'm looking into buying studless snow tires for my wife's 2010 Tribeca.

Stock tire size: 255/55 R18
Snow tire size: 235/55 R18 (I cant get these for an insanely good price)

Overall diameter change: 0.8"

How will this change affect her car?

I'll also be buying a fifth one of these as her spare during winter months (avoiding different tire make/model/size issues on AWD if the spare is needed).

I've done some digging while waiting for a reply...

Very good tire comparison tool: https://tiresize.com/comparison/

This site, and other places I've read, say that +/- 3% of OEM diameter is acceptable.

As long as you keep all the tires the same it shouldn't matter much.