Hey guys,
 
Im having a bit of an issue figuring out about this...
 
Let's say i'd buy an AW11 with stock wheels and i want to get bigger wheels so that i can put low profile tyres on them. How do I figure out the right size of wheels tyres to keep the overall size (wheels + tyre) same as stock to keep factory handling (which is, corect me if Im wrong, best) ?
 
Same questions if i buy a car with upgraded wheels, how do i figure out if the overall size is same as stock.
 
Thank you.

Seems the site is down for
http://mr2.gordaen.com/wt_calc.php

Rule of thumb is you can only go 2 sizes bigger and still be legal
so 14 inch can go to 16 inch and still be legal
when you go up in size for the wheel the size of the tyre (profile will be adjusted accordingly) so that the same rolling diameter is retained, and your speedo wont be out.
this is why the stock wheels have really bulging fat tyres (eg - 60 profile sidewall etc) and  when you fit bigger wheels you will get skinnier looking, low profile tyres (eg - 40 profile sidewall)
 
hope that makes sense.
the above link posted will tell you what size will fit etc its a very helpfull link.
cheers

Your rolling diameter is fairly easily calculatable if you know your tyre size (XXX/YY/ZZ)
Roughly speaking (obviously real world there will be some variations)
Your rolling diameter = XXX*YY%mm + ZZinch
 
So if you got up a few wheel sizes (ZZ), then you need to find the right xxx/yy ratio to keep the same or atleast similar rolling diameter.
 
E.g rolling diameter of a 195/60/14 = 117mm + 14"
rolling diameter of a 215/40/17 = 86mm+ 17"

Touge Charmer,
Google "Miata  tyre calculator". Once into site you put in your current tyre details and your proposed size upgrade. It then clculates the difference in rolling diameter and speedo error. Very handy little tool.
cheers
Steve

Thank you all so much !