E46 18's - 225/40 or 215/35
Discussion
Surely the tyre size will depend on what you had originally on the car and on the width of the wheels you're putting on - otherwise your speedo will be wildly over/under reading.
There's also a 5% difference between a 215/35 and a 225/40.
If it's a 7j rim then 215/225 is fine, if it's an 8j rim then 235/245 is fine, if it's a 9j rim then 245-265 will be fine.
Try this tyre calculator to see which size is closest to your originals http://www.miata.net/garage/tirecalc.html
There's also a 5% difference between a 215/35 and a 225/40.
If it's a 7j rim then 215/225 is fine, if it's an 8j rim then 235/245 is fine, if it's a 9j rim then 245-265 will be fine.
Try this tyre calculator to see which size is closest to your originals http://www.miata.net/garage/tirecalc.html
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