This calculator should help you decide - even shows a drawing of the wheel widrh and diameter changes.
http://www.miata.net/garage/tirecalc.html

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John

2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)

Click to see 87 300E