BMW X1, can I change from Ratio 45 to cheaper Ratio 40 tyres?

Question from visitor, 12/01/2017 :Hi,
I own a 2013 BMW X1 2.0D XDrive, which drives on Run Flats, 225/45 R18 91V. I have noticed when researching replacement tyres that the Ratio 40 tyres are much cheaper than the Ratio 45 which are currently on my car. The question is, could I replace all 4 tyres with the 40 Ratio and if so what would be the disadvantages of doing so?
Thanks
Mike.
Answer by rezulteo, 12/01/2017 :

Hi Mike,

Thanks for your question and apologies for the delayed response.

Legally it isn't possible to replace ratio 45 with ratio 40 tyres. Similar dimensions are no guarantee of compatibility.

Hope this helps, don't hesitate to get back to us if you need further information.

Best regards,

Team Rezulteo