Tires: Tyre Size, tire placard, tyre size

Question
I put narrow and wide tyres on my Audi S5 Quattro. 245/35/19 in the front and 255/35/19 at the back is this fine as this is a four wheel drive

Answer
Leon,

On every vehicle sold in the US – and I am told this is worldwide, but I have not been able to verify this - there is a sticker – commonly called the tire placard - that lists the original tire size and the proper pressure for that size.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

BTW, it doesn't matter who makes the tire or what pressure is listed on the tire's sidewall, if the tire size is the same as the placard, then the pressure listed on placard is also appropriate.

My books are all US fitments only but they say an S5 uses 255/35R19's in all 4 positions.

And what I know about all wheel drive systems, all 4 tires have to be the same diameter or the drivetrain may fail - and to be doubly sure the standard recommendation is to use the same size, make, model, and state of wear.

So you need to get your tire situation resolved ASAP and hope you haven't ruined the drivertain.