What Indigo is saying is your speedo itself is working exactly as designed. Essentially what you've done when you change the tire size in the back is you've changed one of the size of one of the "gears" that feeds the speedometer. Changing the gear size makes it either spin faster (a smaller than stock wheel + tire) which will make it read higher than your actual speed, or spin slower (a larger than stock wheel + tire) which will make it read lower than your actual speed. When this happens, it is a percentage out, not just a flat "this many mph". In other words, the faster you are travelling the more it will be out. If it's off by 5mph @ 50mph, then it will be out by 10mph @ 100mph.
Here's a calculator to show you exactly how far your speedo will be out by plugging in the before (stock) tire size and after (current) tire size.
http://www.miata.net/garage/tirecalc.html
Stock tire size is 225/50/16 or 205/55/16 depending on the option from what I see, the two are within 0.1% of eachother so the difference there is irrelevant.
