To be fair, I should mention that the larger wheels (aluminum rims) were actually heavier than the factory steel 13" wheels by about 8 pounds each.
Also, I found this calculator that will tell you how your speedo will misreport based on wheel size changes:
http://www.miata.net/garage/tirecalc.html
or:
http://www.miata.net/garage/tirecalcold.html
When I put had the larger 195 wheels on my car, my speedometer was slow by .02% (which I consider acceptable).