tire size question

There's a great Tire size calculator at:

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

You need the latest version of Java to use it.

According to this calculator: with 175/65 your speedo would be 1% too slow.
When your speedo would indicate 60 you would actually be going 60.6MPH

With 185/65 your speedo would be 3.2% too slow.
When your speedo would indicate 60 you would actually be going 61.9MPH

With 185/70 your speedo would be 6.4% too slow.
When your speedo would indicate 60 you would actually be going 63.8MPH

You can try it with other sizes.

Edit: Typo

Stock Tire
Sidewall 4.4 in
Radius 11.4 in
Diameter 22.7 in
Circumference 71.4 in
Revolutions/mile 887.0

175/65/14
Sidewall 4.5 in
Radius 11.5 in
Diameter 23.0 in
Circumference 72.1 in
Revolutions/mile 879.0

OR 1% Difference.