well, your speedometer is showing you a different miles per hour than your actually going... first you have to figure out how much its off... go to this site and enter your stock tire size and then compare it with your size tire on your 22's.
http://www.miata.net/garage/tirecalc.html
its deffinalty not normal. how old is your motor? how many miles does it have? you may need a serious tune up .. something may be restricted.