The Miata dot net tire size calculator is a useful tool.
By using it and slightly adjusting tire size, I was able to save $100 on my winter tire package. The tire sizing specification differences were negligible.
However it is a Java Runtime-based application. There are security risks associated with using Java in a browser, and you will probably have to download Java from Oracle. OS vendors (Apple, Microsoft, etc) do not recommend enabling Java in the browser or even downloading and installing it.
Now, I personally have no issue with installing Java on my machines, because Oracle is a trusted vendor and you can disable Java in your browser preferences. So, you could enable it to run the tire size calculator and then disable it, which is a safe way to use it.
There also could be applications on your computer that use Java. Again it's a matter of trust … if you trust the app it's OK, if you don't it's not.
The OS vendors recommend on the basis that you don't have a clue what you're doing. So they do not recommend Java. But it is safe if properly used and configured.
Just a heads-up.
Also note that people confuse Java and JavaScript. There is nothing in common with the two except an unfortunately similar name. Allowing JavaScript in the browser is OK; allowing Java is not unless you trust the page you're using it with.