The primary factor limiting the maximum speed of a Miata is aerodynamic resistance. This resistance is related to boundary layers along the surface of the Miata in which the air is being dragged along. Going downstream, this air forms a wake behind the Miata. The wake is the primary cause of resistance, [see 3, pp. 570-571].
The wake is always relatively wide because the boundary layer separates from the surface at some point, [5,6,7]. This greatly increases the aerodynamic resistance. Controlling separation remains a difficult problem, [4].
There is also the problem that the boundary layer is normally turbulent. By itself, turbulence will increase drag due to its thermodynamically irreversible mechanics, [9]. However, often transition to turbulence decreases drag instead because it also tends to delay separation.
The high speed of, in particular, white Miatas, will also bring in compressibility effects, [1]. Note that such effects are largest in elevated speed areas such as near the top of the windshield header.