I'm looking into buying a set of 215/55/15 winter wheels and it occurred to me that my spare is 235/40/18 with a summer tire mounted.

I'm under the impression that the quatro system likes to have the wheels on each axel near identical to each other otherwise it can damage the system.

Does anyone know what the implications would be if I blew out one of the winter tires and had to run the spare for a time?

 

Tire Diameter Difference is 4.3%. Not great.

Wow, 15's on a B6 A4 - you're a brave man

 

Whoops, meant 16!

215/55/16

 

Tire Diameter Difference is 0.36%. You're cool.

 

good news!
I wasn't liking the idea of having a fifth winter wheel.

Of course, when I asked the dealership about it they said it would be best to buy a fifth.. Best perhaps, but worth it? Seems not.

 

Don't buy a 5th wheel. It's just there as a spare. Look at what most cars run as a spare - a tiny donut wheel! I would say though that you would be better to get your winter tire back on as soon as you can have it fixed (in the event you needed to) for peace of mind and better winter traction.

Besides, you don't want to drive around in a beautiful car with one mismatched wheel!