Iv got a set of 20s on my a8 they have 245/40/20s tyres on , i need to replace them as missing one alloy iv found these Q5 wheels that have 255/45/20 on them would  the alloys fit with either the tyres that’s on there or changing them all round 

If the width is the same then you should be fine. Probs have to have a lower profile tyre though 

James,

What year of Q5 are the wheels off?

I can check the PDC, Offset etc

 

for instance the 2.0tfsi Q5 2008 - 2012 has an offset of 37 - 41  where the A8 3.0 Tdi 2005 has an offset range of 43 - 47.

As far as i remember (i can double check later) the OE 20's came with 255/35R20 ET43 (the 18s were et45 & 17s were et43)