Good [insert applicable time of day reference here] you fancy people here at civicforums. It's your neighborhood friendly Riceboy admin here. I just thought I'd share an awesome tire/rim calculator I found online a while back, and I just kept forgetting to share with you guys.
https://www.wheel-size.com/calc/
Now, the old miata.net calculator that many of us have used for a while was fantastic, but this one is better. It just provides so much more information, but is a bit more involved. You can, by all means, use it like the miata.net one to get effects on speedo reading (it delivers that information by a percentage change), but you can also input things like how offset affects various parameters like fenderwell clearance, caliper clearance, or even scrub radius. Also gives a breakdown of "this is why it will/won't fit" kinda deals. Hope this helps you guys with your wheel/tire choices!
-Josh
