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

use this to be exact

a tad too slow means the speedo is lower than what you are actually traveling. which could be bad, since sometimes cops won't pull you over for 79-80, but 81 then will. you could think your going 80 when you are actually at 81-82. then again the speedo is not 100% accurate on stock tires anyways.