It is recommended to keep the MAXIMUM difference in diameters to �2.5%
:k49:
175/65 R14 Benchmark

185/55 R15 +0.34% 2mm
195/50 R15 -1.2% -7mm
195/55 R15 +2.23% 13mm
205/50 R15 +0.51% 3mm
225/45 R15 +0.17% 1mm

195/45 R16 -0.17% -1mm
205/40 R16 -2.23% -13mm
205/45 R16 +1.37% 8mm
215/40 R16 -0.86% -5mm
225/40 R16 -0.51% 3mm

195/40 R17 -0.86% -5mm
205/35 R17 -1.37% -8mm
205/40 R17 +2.23% 13mm
215/35 R17 -0.17% -1mm
225/35 R17 +1.03% 6mm

205/30 R18 -0.51% -3mm
215/30 R18 +0.51% 3mm
225/30 R18 +1.54% 9mm

If the size is not available in the list, please calculate yourself using the following link

http://www.miata.net/garage/tirecalc.html