Go here:

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

Or here:

http://www.1010tires.com/TireSizeCalculator.asp

This one will give you a range of tires you can put on your car and
maintain the right speedo reading:

http://www.mr2.com/FORMS/tire.html

This will give you tire sizes and sidewall heights:

http://www.rojawheels.com/wheeltech/DiameterCalculator/DiameterCalculator.htm

And read this:

http://www.content-articles.com/article.aspx?i=40357

I put a wider set of tires on a car than the car would accept. But, I
remembered this making turns, and the tires would only hit the frame at
full lock. In that car, full lock was about a 90 degree turn! So I wasn't
too worried about it...

But, there are other things, too. Tires of one size fit on rims of a
certain width. I wanted to put 195/45-14s on a Corolla, but I need a 1"
wider rim (I had the rims on a Celica that would work, but gave the car
away...). Otherwise, the tire may blow off the rim in a hard turn.

Also, check the clearance of the flanges that hold the springs up. A tire
with too much of a diamter may not clear, or rub when going over bumps.

Let's say your original tire size is 205/60-15. A wider, lower profile
tire will give you better handling. In this case, keeping the original
rims, then you could probably go 225/50-15. You'll get decent handling,
good ride and save about $1000...

That is, if $$$ is an object!