Before I asked, I checked the 205/55R16 and 215/50R16 on the Mazda Miata site's tire calc, and the 215 works out exactly the same diameter, while the 205 will be off by about 1mph at 60 (certainly close enough). That said, diameter isn't the question as much as clearance between strut and sidewall.
I can get my middle-finger between the sidewall and strut as it is now with the Michelin 195/55R16s that are on there, which suggests I may be able to go a little wider and not rub.
Any anecdotal info out there on this?
Thanks!
C ya,
Dutch