The only way to alleviate that is to get a new tire with as close to the same overall diameter as the oem tire.
use this to find out what tire size you should go with
http://miata.net/garage/tirecalc.html
for example, my oem tire size was 185/65-15, and the closest 17" tire would be a 215/45-17, which would make my speedometer read .6% slower than my actual speed (with a 60mph indicated speed, I would actually be traveling at 60.4mph)