Great web site and service. I have 8/2011 BMW 740Li RFT on 20" rim. I am looking for more ride comfort/ softness and quietness. The car is in excellent condition.
I appreciate answering below questions to assist me making the right purchase from your site:
1- Can I lower the rim/ wheel size from 20" to 18" to have softer ride? Or 19" is best?
2- If answer above is yes, is there going to be impact on any part of the car or performance in the future (e.g. Suspension)?
3- Is dis-comfort I'm feeling because of RFT type or 20" wheel size?
The concept is I want to increase the rubber to increase the comfort. Any advise to resolve ambiguity is really appreciated.
To answer your first question, reducing your wheel diameter and increasing the sidewall height will offer a more comfortable ride. To select a smaller wheel size you will need to know what diameter will clear the brake calipers. Looking at the fitment options on Tyresales.com.au for your car it shows an 18" wheel, 245/50R18 tyre size, as a standard fitment so this could be an option. I would suggest to double check with BMW first as you will need to purchase new rims so it is best to be 100% certain.
Secondly, when you go to a larger profile tyre the main thing you will notice is a reduction in steering response. There will be no other affect on your steering or suspension as long as the replacement tyres have the right outside diameter and width.
Thirdly, most of the harshness of your ride would be from the low profile 20" tyres. Run flat tyres (RFT) do have a stiffer sidewall than normal car tyres but if your car comes standard with RFT the vehicle manufacturer would have accounted for this during its design.