Easiest way to do it is, go to the following link,
fill out the details as far as what size tyre you are running on the car as well as the size of the temp spare.
It will then calculate the rolling diametre of both tyres and give you a comparison on the difference in size between them.
This will then tell you if the temp spare is suitable or not in percentage.
For the temp spare to be suitble, the calculation should give you a difference of between
98% to 102%. (A tolarence of roughly 2mm either way)
Anything greater I wouldn't realy recommend using.
http://www.miata.net...e/tirecalc.html
