You can use this formula to calculate diameters of tire / wheel combinations. Of course, the offset of the wheel, and whatever else may hit the wheel and tire limits choices (usually the front wheel / tire since it has the most "stuff" to hit while at full lock,left or right)
You can increase the wheel/tire size without messing up the overall outside diameter.
I’ll give you an example on how this is done and how it can be calculated for any dream combination you can think of.
Stock wheel and tire 185/70-13 (2nd Gen). 185 (tire width in MM) X 0.70 (aspect ratio)= 129.5MM X 2 (since the tires on both sides of the rim)= 259MM / (divided) by 25.4 (to convert MM to inches)= 10 inches + 13 inch rim dia. = 23.197inches Overall Tire Diameter.
Now, lets use the 205/40-17’s on my car.
205 X 0.4= 82 X 2=164MM / 25.4= 6.457 inches + 17= 23.456 inches Overall Tire Diameter.
Soooo, you can get a much shorter sidewall, larger rim diameter while not changing the speedometer or final drive gear ratio.

 

OR...insted of breaking out that calculator and stressing your brain....go to:

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

and you have a handy dandy tire size calulator that all you do is put in your STOCK tire size on one side...and then select the tire size you are considering and it tells you MANY deminsions of both your stock tire AND the one you are considering...the sidewall height, the Radius, the diameter, the the circumfrence, the RPM (revolutions per MILE) of that tire.....and it will also calculate the speedometer error of your car based on the difference of the stock tire and the new one selected and tell you the difference in the speedo reading at 60 MPH as well.

Personally I am lazy when it comes to these things and this automatic calculator is much faster than crunching numbers.

 

hey... Thanks for the link, it does make it easier. You know what they say, give a man a fish, he'll eat for today, teach him to fish...

 

and he will spend all his money on a boat, and spend all his time on the water!! :{D