Here's the deal. Currently I have an 88 SE and am still using the 4-lug 14" tires.

Stock: 185/70/14

I want to change the size, but was wondering which size is closest to the stock diameter for speedometer purposes.

215/60/14
225/60/14

215/65/14
225/65/14

I know there is a formula somewhere for this kinda math, but hell if I know where it is!

Thanks.

 

[URL=http://www.miata.net/garage/tirecalc.html]

 

www.miata.net/garage/tirecalc.html
(operator trouble, oops)

 

the 185 is the width in mm (W), 70 is the sidewall height as a percentage of width (S), 14 is rim diameter in inches (D).

The formula I use is ((W * .02S) / 25.4) + D to get an answer in inches.

Since you're staying with 14" rims, all we need to know is the sidewall height - which is W * .01S.

185/70 has a 129.5mm sidewall height.
215/60 has a 129mm sidewall height, or a 1mm difference in diameter. Tire tread depth is a bigger factor than that, as is inflation pressure. Most manufacturers interchange 185/70 and 215/60 freely.

225/60 is 135mm sidewall height. (11mm diameter diff)
215/65 is 139mm sidewall height. (19mm diameter diff - that's 3/4 inch!!)
225/65 (they make this?) would be so much bigger I won't bother doing the math.

BTW - this was the first formula I ever figured out - I didn't even know what formulas were at the time. I just knew how to multiply by a percentage, and how to convert MM to inches. I used to caculate tire diameters for FUN. (Hey, everything is interesting when you're 5 years old )