Is the spare supplied to the Americas different to those supplied in the UK? We have steel spare wheels slightly slimmer than the regular wheels:
UK law requires them to be fully inflated - or not present at all or else they are an MOT failure point.
Curiously it is debatable as to whether they are actually legal to use - because using different sized tyres on the same axle is illegal. The speed is limited to 60 mph according to the sticker on the wheel. I've never used mine but in a get you home scenario most certainly would.