Here is a chart created to show equivalent metric sizes that correspond to the older bias ply tire standard. Accuracy may slightly vary with the conversion. The tire diameter is the important issue regarding speedometer reading to vehicle speed when switching tires. There is also an excellent graphic tire size calculator showing two different tires and their comparison at Tire size calculator, compliments of the Miata.net website. You can see the width and height of two tires against each other.
 
Standard Metric
Overall Wheel Diameter 78 series 14" Bias Ply 70 series 14" Bias Ply 60 series 14" Bias Ply 78 series 15" Bias Ply 70 series 15" Bias Ply 60 series 15" Bias Ply 75 series 14" Radial 70 series 14" Radial 60 series 14" Radial 75 series 15" Radial 70 series 15" Radial 60 series 15" Radial 50 series 15" Radial
21.8" - - - - - - - - 165/60 R14 - - - -
22.3" - - - - - - - - 175/60 R14 - - - -
22.7" - - - - - - - - 185/60 R14 - - - -
23.0" - - - - - - - - - 135/75 R15 - - -
23.1" - - - - - - - 165/70 R14 - - - - 205/50 R15
23.2" - - - - - - - - 195/60 R14 - - - -
23.6" - - - - - - - 175/70 R14 - 145/75 R15 - - -
23.7" - - - - - - - - 205/60 R14 - - 185/60 R15 -
23.9" - - - - - - - - - - - - 225/50 R15
24.2" A78-14 - - - - - - 185/70 R14 215/60 R14 155/75 R15 - 195/60 R15 -
24.6" - - - - - - - - 225/60 R14 - - - -
24.7" B78-14 - - - - - - 195/70 R14 - 165/75 R15 - 205/60 R15 -
24.8" - - E60-14 A78-15 - - - - - - - - -
24.9" - - - - - - 185/75 R14 - - - - - -
25.1" - - - - - - - - 235/60 R14 - - - -
25.2" C78-14 - - - - - - - - - 185/70 R15 215/60 R15 -
25.3" - - F60-14 - - - - 205/70 R14 - - - - -
25.4" - - - B78-15 - - - - - - - - -
25.5" D78-14 - - - - E60-15 195/75 R14 - - - - - -
25.6" - - - - - - - - - - - 225/60 R15 -
25.7" - - - - - - - - - - 195/70 R15 - -
25.8" - E70-14 G60-14 C78-15 - - - - - - - - -
25.9" - - - - - F60-15 - 215/70 R14 - 185/75 R15 - - -
26.0" E78-14 - - - - - - - - - - - -
26.1" - - - - - - 205/75 R14 - - - - 235/60 R15 -
26.2" - F70-14 - D78-15 - - - - - - - - -
26.3" - - - - - - - - - - 205/70 R15 - -
26.4" - - H60-14 - E70-15 G60-15 - 225/70 R14 - - - - -
26.5" F78-14 - - - - - - - - 195/75 R15 - - -
26.6" - - - E78-15 - - - - - - - 245/60 R15 -
26.7" - - - - - - 215/75 R14 - - - - - -
26.8" - F70-14 - - - - - - - - - - -
26.9" - - - - F70-15 - - - - - 215/70 R15 - -
27.0" - - - - - - - 235/70 R14 - - - 255/60 R15 -
27.1" G78-14 - - - - H60-15 - - - 205/75 R15 - - -
27.2" - - - F78-15 - - - - - - - - -
27.3" - - - - - - 225/75 R14 - - - - - -
27.4" - - - - - J60-15 - - - - 225/70 R15 - -
27.5" - H70-14 - - G70-15 - - - - - - - -
27.7" - - - G78-15 - - - - - 215/75 R15 - - -
27.8" H78-14 - - - - - - - - - - - -
27.9" - - - - - L60-15 235/75 R14 - - - - - -
28.0" - - - - - - - - - - 235/70 R15 275/60 R15 -
28.1" - - - - H70-15 - - - - - - - -
28.2" J78-14 - - - - - - - - - - - -
28.3" - - - - - - - - - 225/75 R15 - - -
28.4" - - - H78-15 J70-15 - - - - - - - -
28.7" - - - J78-15 - - - - - - - - -
28.9" - - - - L70-15 - - - - 235/75 R15 - - -
29.1" - - - - - - - - - - 255/70 R15 - -
29.3" - - - L78-15 - - - - - - - - -