My 1999 SL500 has exactly the same spare wheel and tyre. The tyre is the same size as the tyres on the car (245/45 R17 at all four corners) and there doesn't appear to be anything special about the wheel. I guess it is cheaper than fitting a regular spare. I also think you'd have this spare if you spec'd the car ex-works with 18" wheels, but you'd have to keep it to 50mph in use.

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190