Spare wheel size

[QuantumJohn]

I've doen a search for this but can't find it mentioned anywhere.

Does everyone buy five new wheels when they upgrade the size?
Before adding 18" wheels to my B5 S4 I want to know if I shoudl carry a spare of the same size as surely you should never run a different sized wheel/tyre with Quattro or does the 225/45R17 and 225/40R18 tyres give the same rolling radius?

Thanks,
John

[SimonS4MTM]

They are a similar roling radius.

If you only use the spare as an emergency then i dont think it will cause any problems if it is a different size.

[BRETTCOLLINS]

I have a full size spare in my S4 B5 18in Alloy with 225/40zr18 tyre. It just fits.


hope this helps

[Dippy]

Don't forget that even if the spare wheel is the same size, the tyre wear is unlikely to be the same anyway.

I recently did a lot of reading about 4 wheel drive transmissions, including the TORSEN differential, and I found nothing about any danger from using different wheels on the same axle. I have heard people say that you shouldn't have different tyres, but never any technical evidence/explanation to back up the warning.

That's not to say that I would run different tyres for long. I have 18" wheels but a 17" spare. If and when I get a puncture, I will get it fixed asap and run the car gently until then. But as I say, this is just out of paranoia, not because of anything certain!

[dog]

How do you fit a 17 rim to the front? Or do you change 2 tires for a front puncture?

[TWH]

The only reason you shouldn't have different tyres across an axle is because they will have different construction and tread patterns. This means that they will perform differently and will wear differently, so one tyre is likely to take far more strain than the other, and if you've got one side of your car gripping and the other not, then it could get quite hairy. That's my understanding of the whole thing anyway. Similar issues with different size wheels, will just affect the stability of the car, shouldn't affect the transmission, that's why there are differentials in place.