Well I just went and measured the wheel in a '93 C180, (it's the standard airbag style wheel) and it's 390mm (outer diamter) too.
Whilst I thought it would not be much different the older 410mm wheel, it does actually feel quite different - noticeably smaller and without the bus-like impression. I think it will suffice
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I then jumped into an E30 BMW M3 next to it, which had an aftermarket 360mm wheel. I felt that this was a little 'go-kart' like for what is not a very small car, even if it's a very sporting model.
Certainly a 360mm wheel on a 190E would seem rather out of place on any 190E except for an Evolution or at least a 16v.
So that's me done. Thanks all for the help.. off to find a sportline wheel!
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I'll post pictures when I've got it.
Russ