Sorry to bring up the subject again, but there is some things I don't understand!.
I have the staggered set up so 255 front and 235 rear.
My understanding is the hight of a tyre is the % of the width, so taking a 255/30 and replacing it with a 245/35 is a 18,5mm larger circumference 2.83%
235/35 are ok all round as they are the same!
I thought Audi made the different front size on the staggered it improve handling, also timed over the co2 output too lol.
To the question is the Pirelli ro2 a special size tyre? why does it differ in size (Width) to the Michelin ?
Yes it is a complete buggers muddle. There is no tyre that fits well on an 8.5" rim that is close enough to match the 235/35 size. You would need a 265/30 which is a bit too big for an 8.5 rim. That is why Audi had Pirelli make the RO2. The problem is that it has not much grip and seems to wear the inside edge. I have had the tyres next to a few different standard ones and measured them.
The Pirelli RO2 tyre is 255 but that includes the massive 'rim-protector' the tread is much narrower (about 240). That 'rim-protector' looks to me to be a stiffener under hard cornering to support the edge of the tyre as it moves away from beneath the rim. Although it helps with the understeer I think it wears the inside corner where the tyre does not give way because of the stiffener, poor grip in a straight line because of the stiff side walls and traction light because it does not behave the same as the (more normal construction) rears and can wear more causing the TC light because of the fluctuating circumference difference. The update made the TC less sensitive.
This update also helps the staggered Michelin because the circumferences are not matched like the Pirelli.
People will argue whether this is a problem or not but personally I would not run a big difference in rolling radius on a haldex car as it may not show for many miles.
The Pirellis on mine had worn on the inside edge to the canvas, I had poor grip and TC light issues. I changed 2 staggered sets of wheels into 2 square sets, 235/35 on 8" for winter and 245/35 PS4S on 8.5" for summer. I went with 245/35 as it is a good fit on the 8.5 rim, bit more sidewall comfort (standard TTRS size) but 255/30 PS4S all round would also work on square 8.5" and I may try that next time.
I get even wear on both sets, no TC light issues (no update) and no increase in understeer but I also think the geometry change at the front of the saloon along with more weight further over the back wheels helps with this more than people give it credit.
For me the handling on the square set of 8.5" wheels with 255/35 PS4S is sublime and good ride quality on the crappy roads here. IMHO how it should have come from the factory.
. 
but am wondering if anyone knows if there are any decent tyre deals around.
