There are lots of websites that have a Tire Size Calculator. Some can compare two tires at a time. You'll find links to various websites that contain tire size calculators to help you out. See what happens to your speedometer when you change tire size etc.

To compare two tire sizes, select the sizes, then click 'Compare Tire Sizes'.

This will generate comparative information on diameter, radius, sidewall height, circumference, RPM and speedometer difference.

Info:
Tire diameters can vary slightly for each tire model. The listed diameters are from calculations based on the tire size.

When changing tire sizes, we recommend staying within 3% of the diameter of the original tire. Any more than this and you face the risk of brake failure.

This calculator is for information purposes only and we do not guarantee fitment based on this calculator alone.

Miata

Calculator 1:
Use the calculator to see what happens to your speedometer reading when you change the size of your tires and wheels. The graphic shows your stock tire selection in an outline overlaid on your new tire selection.

Calculator 2:
Their other calculator gives various tire measurements from standard tire size specifications.

It gives several dimensions for the tires listed in both fields. The output will also show the tire specs given in the "Tires to compare" field as a percentage difference from the tire spec in the "Stock size" field.

If this description sounds confusing, then just ignore it and try the calculator. You'll get the idea quick enough!

DSM

If you are contemplating changing the size of your tires or the size of your wheels, but still want the speedometer/odometer to be correct, just enter the size of the tires that were originally on your car and I will give you some options.