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It depends on two things:

1) What you're trying to optimise
2) The rim width

Obviously you want to keep the rolling diameter as near standard as possible to minimise speedo error. If you're trying to optimise cornering grip, you need to fit one of the narrower tyres that will fit on that rim width, to reduce tyrewall flexing. If you simply want to get as much rubber on the road as possible, go to the other end of the scale, although all this will give you is (perhaps) more straight-line traction at the expense of cornering grip, although all that sidewall flexing will also give a softer ride.

Andrew's Aero rides quite softly on those 225s.

For speedo accuracy, your choice is probably limited to:

215/45 (1% fast)
225/45 (0.4% slow)
235/40 (1.9% fast)
 

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Depending on offset, anything 225 or above may run in to problems with rubbing on the inside of the front arches on full lock and outer of the rear when fully laden, especially if you have lowered suspension.

I has 225/50R16 on Aeros and it occurred with them. I now have 215/45R17 and no probs on 7J wheels. FWIW I reckon this is the optimum tyre size for CS's.
 

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205/50R17 would be OK... speedo reading would be circa 0.8% too slow. I just don't think it's a very common size, so could be hard to find and expensive, whereas there's a lot of choice in 215/17R45.

The upside however of the 205/50 is they generally have a slightly higher load rating of 89, whereas most of the 215/45s I've seen are 87

(the load ratings above apply to high performance tyres)
 
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