I'm another victim of the spare wheel thieves... I'm running on 18" wheels. I called a scrap yard who said they'd send me a 18" wheel. Instead they've sent a 19". Instincts tell me that this might cause problems if used as a spare but as I'm not that technical I wondered what forum members thought?

I would think that as long as the rolling radius is the same then you should be ok.

There was another discussion recently regarding the factory standard 19" spare for the cars running on 20" rims.
http://www.rrsport.co.uk/forum/topic25528.html

The factory standard spare actually has a rolling radius larger than the rest of the wheels - but its my guess that the sidewalls compress on the narrow tyre to give an overall circumfrence of the same.

Hope this and the above thread helps  RRS 3.0D HSE 11MY - Fuji White, a few extras.

That's really handy, many thanks.