The sticker inside the fuel cap lid shows that a normal tyre pressure is 2.3 bar but ECO is 2.6 bar. How significant is this difference in terms of mpg savings? Is the ride noticeably firmer with 2.6 bar compared to 2.3?

33.3 against 37.7 psi. I think that would be pretty firm but not uncomfortably so. I used to run my Alhambra rears at 42psi when towing our caravan and wouldn't drop them whilst away and didn't have any issues. Haven't tried it on the S3, but it is on my list of things to do.

I run my winter tyres at 2.7 bar (winter tyres run at 0.2 bar higher than normal tyres due to their compound etc) without any issues and with the DCC in sport as well.

 

The higher the pressure the less rolling resistance the tyre has so this has to have an effect on the MPG

I have just switched back to the normal setting from eco. I couldn't see any real difference in mpg and the ride felt a touch more jittery. (subjective judgement of course)

 

Ran for 1000 miles on eco pressures. 150TDI