Seriously, how to cause extra stress? Got see what rallykid
said? He say his car has 17" and his spare is 15"..tell me is a 15"
rim even lighter than a 17" rim? If it is, how will a 15" spare
tyre cause extra stress on drivetrain? 
Added: And just like what n0x said, who will change to the 15"
spare and drive it for long term? Obviously is ASAP go to tyre shop
to change a new 17" tyre and put back the 17" rims..the 15" spare
only temporary what..