215/40/18

[roogz9]

will a 215/40/18 tire size mess up my speedoo at all?

[jayson_waltz]

http://www.miata.net/garage/tirecalc.html
That will tell you everything that will happen when you changing sizes.

[CBLOHM3400]

Great Site

[AlbinoMonkeyRat]

yes, that is a good site. 215/40/18 is a decent comparison...probably just as close as you'll get to stock size without actually using a stock size.