Spare tyre size info

[stargate]

I've just checked that my spare tyre is actually smaller than my original tyre. The spare tyre is 13" while my tyre is 15" size. Is it ok to use it or I need to replace it with the same size?

 

[Doink]

u can...temp use mah...
15 inch with 50 series tyre match with 13 inches 70series tyre...

 

[AvexTrax]

but...if the disc brake size only fit for 15inch rim..how?