Check out this page from Miata.net:Tire Size CalculatorHandiest dang thing! Right down to circumference so you can go for maximum stuffage in those wells! You should pay attention to the width though, and probably not exceed 205mm (Maybe 215 ?)
I have a set of Volvo Hydra's which are 6.5in by 16 in with 205 wide tires on my 87 745T and the fit is fine. Hydra's are the swept 5 spokes. If they are from a Volvo RWD they will fit fine. The only exception is the 95-98 960's had a different off set simular to the FWD 850's and S70's.