Hi guys I just recently bought a 97 318ti and I wanted to say hello.
O.k. it seems to me there are allot of tire sizing questions for our vehicles and I just wanted to contribute some how. I didn't see one posted.
I have been using this calculator for years on my 5 series as well as other vehicles. Hopefully this helps those that can't figure out what size tires to run with there new wheels.
If you dont know how to use it, it is dummy proof. Just plug in what you have for OEM (wheel size and tire size) and plug in your new wheel and you can see what tires would keep you at stock (rolling diameter) and what will be big or small. Also you will know how much your speedo will be off or on by the wheel upgrade.
http://www.miata.net/garage/tirecalc.html It also helps by giving you dimensions for those of us who are lowered. By using the height added or loss you can see how much is too much or too little.
There's more to wheels and tires than just this. But this should help get people started.
Tire size calculator 
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