Default A6 C6 - downsizing to 18" OEM wheels - use 245/45 R18 tires

Hi Guys
My question is regarding 2009 Audi A6 3.2L Quattro (it’s 2009 but production of 2008 so it’s the pre-facelift) with factory installed 255/35R19 tires and 8.5jx19 ET48 wheels. The car also has option 1BE which is the sport suspension.

I just bought the car last week and I must change the tires because they are in terrible condition. The original owner said he had difficulty finding this tire size and upon searching at the main traders, I also didn’t find it. I also think the wheels might have some defects because there is vibration and noise above 100 km/h. Roads here also have many pot holes or horizontal lines so the ride isn’t really nice with such a heavy car and low profile tires. I am currently driving an A4 B8 with standard suspension ad 225/55R16 tires and to be honest I don’t feel the “comfort” of the A6 at all !

For that, I will buy new 18” wheels as the 18” tires is commonly available here for BMW and Mercedes Sedans. My current wheel is 4f0601025BA with ET48. I bought the wheel 4F0601025AL (which is 8.5j ET46).

The tire size available widely here is 245x45 R18, because it is used on the 5-series. The car catalog indicates that at 18”, the tire size is 245x40 or 255x40. I made some calculations and came up with the following :-
The outer tire diameter of my 255x35R19 : 661.1mm
The outer tire diameter incase of 245x40R18 : 653.2mm (less) - not available
The outer tire diameter incase of 255x40R18 : 661.2mm (exact match) - not available
The outer tire diameter incase of 245x45R18 : 677.7mm (little more) - available easily
My questions are :-
A- Has anyone tried the 245x45R18? Would it fit? Do you have pictures?
B- Is there a concern about rubbing? The addition in diameter is 677.7 – 661.1 = 16.6mm
C- Will I be at risk to damage anything in the suspension or struts by using such a profile (45)? since I am changing the "unsprung mass"
D- What are the differences between the sport suspension and the standard one? Because I am suspecting the A6 must be much more comfortable than this !!

I don't think you'll have any issues with 245/45 18s, but can only base my experience on UK spec cars. I have an allroad (so not a fair comparison), but run 245/45 18s instead of the original 245/40 19s. There's sufficient height in the allroad suspension to allow fitment of 235/55 17s with a diameter of just over 690mm even at the lowest "dynamic" level of the air suspension, but I couldn't comment on yours. Don't forget your calculation of the difference in diameter relates to two sides of the tyre its rare for the 9 o'clock and 3 o'clock positions to be close to bodywork simultaneously. Normally its the top fouling either on suspension or bodywork when the suspension is compressed.

There can also be differences in the nominal diameter of different patterns of tyre despite being generally 245/45 18, so you might prefer to be cautious at the expense of reduced tread depth and working life. I presume Egypt doesn't have as much rain as the UK and that it is perhaps not as strict as us regarding minimum tread depths.

My original offset is ET43 and my other wheels are ET45. Normally a change of offset within 5mm each way is not an issue assuming wheel and tyre widths remain the same. Check willtheyfit.com for a visual representation.

I have 254/40 x 18 tyres with the 1BE suspension, so only one profile size down from the ones you're looking at.

As long as you choose wheels with a similar offset I would say that there's plenty of room left in the arches. This is only a presumption on my part but given that the S line suspension option can take the car even lower, there has to be some extra room in there somewhere!