Richard Murphy ( snipped-for-privacy@murphy00.freeserve.co.uk) gurgled happily, sounding much like they were saying :
There's not a huge difference between 185/65 and 185/70 in size - about 9mm in the sidewall height, or only slightly more than the difference between a bald and new tyre of the same type and size.
Many new cars come with a spare that's a different size to those fitted - not just "space saver" spares, either - but virtually anything with alloys and a steel spare.
Even so, it's not a great idea.