Q3 Quattro 20" rims 18" Spare ???
 

I have a 2018 Q3 quattro. Currently has 20" rims and Pirelli P zero 255 35 R20. I just found the Spare is a Hankook T145 / 80 R18 109M saver tire. If I had a flat on the road, would not this addition of the 18" rim with 3 20" rims cause damage to the differential ??

Any help would be appreciated.

No damage, the outside circumferences of each size tire you listed are almost equal at 27". The 18" spare is only 0.1" larger.
As with all spares, they are only temporary to get you to a tire shop and not be use long term due to the width and speed rating.

LX302, thank you very much. I guess one of these days I will understand the circumference / ratios etc.....but, at 73 years old, I might be too optimistic. Thanks again for that response. makes me feel a lot better because its my wife's vehicle.

Q3 2018 Quattro 20" versus 18" rims
 

LX302, thank you very much. I guess one of these days I will understand the circumference / ratios etc.....but, at 73 years old, I might be too optimistic. Thanks again for that response. makes me feel a lot better because its my wife's vehicle.