I just got a ticket for doing 58K in a 50K limit - my car is an 87 n/a with 15" aftermarket rims - does the change in wheel size from the stock 14" rims make enough difference to the speedometer to give a false reading enough to show me driving at approx. 52K when the radar said 58K - cops suck!

 

The wheel diameter doesn't matter, it's the diameter of the tyre. If you increase the wheel diameter, you need to fit lower profile tyres to maintain the original tyre diameter (or get your speedo recalibrated). What profile are your new tyres? Any wheel/tyre fitter who fits tyres that throw your speedo out by 11% is a ********!
It might just be your speedo though...

 

Try this out. http://www.miata.net/garage/tirecalc
But there will always be a minimal difference because different makes of tires of the same size are actually slightly different sizes.
Go to www.tirerack.com and check the specs on different tires and you will see that two tires of the 'same' size will have different revolutions per mile.