[dfiore]

I went to a rim shop and was told I can for sure use 15x5.5 rims, but it unknown whether 16x6.5 rims would fit or rub.

The, of course, if the issue of unsprung weight, but I could overlook that...maybe

 

[LoNOx 1]

Try http://www.miata.net/garage/tirecalc.html to compare tire sizes and correction factors.