31" tyres on 19" rims have the same sidewall height as 30" on 18" rims.
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I have a similar problem to others with 18" Compomotive rims it seems, in cheaply setting up a second spare. I went for the 285/60R18s and really like their purposeful look. But these of course are a larger rolling diameter than the 265s that most went for, and 5% larger than the stock 255/55R19. So I'm ruling out keeping an old 19" rim, unless perhaps I shod it with a funky tyre like a 235/65R19 which gets me back to 1.6% difference, similar to the difference LRD414 tested as ok.
I think I best get a cheap 18" LR rim (I see one for ~$240 on ebay ATM) which will be ok for the rears, and a two-step for the front I understand. Then I can match the diameter exactly.
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D4 SDV6 MY16 with Llams, 18" Wheels & D697s, Traxide DBS, APT Sliders and compressor cover, OL long range tank, FrontRunner RWC, TowPro Elite