Originally Posted by
Mondego
Another issue to consider is, in order to maintain rolling radius, or wheel diameter, the profile needs to change in direct ratio to the tyre width. Since the profile is given as a percentage of the width. e.g, a tyre with profile 50 (%) with a tread width of 200, would mean the profile equates to 100mm (half, or 50% of the width). So changing one and not the other will result in a change in wheel diameter/rolling radius.
Use a tyre size calculator to ease the headache:
http://www.discovery2.co.uk/tyresize.html
As I mentioned above. Those tyre size calculators are good but sometimes as in this case to keep the rolling radius right you end up with a size that either doesn't exist or is so uncommon the price becomes uneconomical especially as its only temporary until new wheels are purchased
Regards
Matt