A helpful post I found while searching the forum:
Use these two links and make sure you stay below 1% difference f/b.
You should be fine then.
http://www.miata.net/garage/tirecalc.html
http://www.1010tires.com/tiresizecalculator.asp
These are the proven SyTy combos:
245/45R17 - 275/40R17 - 315/35R17 (Vette Style)
245/40R18 - 275/35 R18 or 285/35 R18
235/35R19 - 275/30R19 (also possible 245/35R19 - 285/30R19)
245/30R20 - 295/25R20 (av. only in Europe / alt. for US 255/30R20 Pirellis on all 4 wheels)
Stay away from all the 255/35R20 combos!
225/55 R16 for winter tires - way better handling
You can interchange all of them f. ex.:
245/45R17 in front and 275/35R18 in the back
or even 295/25R20 in the back...