225/50/17 or 225/45/17 ?

Question from visitor, 17/05/2011 :I have 4x 225/50/17 run flat tyres on my car. Can I replace these with 225/45/17 and if so what are the pros/cons ?
Answer by rezulteo, 17/05/2011 :

First of all, your tyre dimension is not the size homologated by your car maker.
With a smaller ratio, the diameter of your wheel has changed, so your speed indicator gives you wrong information.

Please note that, if your car has runflat tyres, there is a chance it can only be equiped with runflat tyres, as is the case with some BMW.
For exemple, the BMW X1 has a homologated size of 225 50 17 94H and 225 50 17 94V, with ZP tyres (run flat tyres by Michelin)
So our advice for your car tyres is go run flat.