Need sound advice on TIRE UPGRADE!

[Raque Thomas]

The 285/55's are 2.6% too slow - when the speedo reads 60, you are actually going 61.6mph. The stock tire diameter is 29.6", these are 30.3". The 285/60's are 6.4% too slow - when the speedo reads 60, you are actually going 63.8mph. These tires are 31.5" in diameter. Check this site for more info:
http://www.miata.net/garage/tirecalc.html

[Moncha]

There's the Miata tire calculator and a couple of other ones listed in the Web Links Section