Hi,
:)
I am wanting to buy some winter tyres that can temporarily be used on on another car that takes 205/55/16 tyres but would also run on 205/50/16 (just). These tyres will be removed in January to put on my Cooper D.

In deciding wether this plan is viable I need to know the factory standard wheel size. Unfortunately there are 4 different "standard" wheel sizes ranging from 608.5mm to 621.2 mm.

205/55/16 winter tyres are a lot more plentiful in choice than 205/50/16 ones. Also the other car has 7.5J wheels that can not fit anything narrower than 205 width tyres.

I would prefer to go large for extra winter ground clearance if possible, so the 205/55/16 (632mm) size would be my choice so long as it is no more than 2.5% greater than "Stock".

But what exactly is "stock"?

Yep, I think 'stock' is a range, not a single number!

2.5% less than your 632mm gets you into the 'stock' range, so I think you're OK. And I don't think there is any hard-and-fast rule about how far out you can go.

My Mini speedo lies so much that worrying about its accuracy is pointless - tyres big enough just to cancel out its error wouldn't fit in the wheel arch....

632mm may be noticeable as a loss of acceleration due to the gearing change, but with the torque of a diesel, it shouldn't be too bad.

Well I have ordered a set of Nokian WR A3 205/55 R16 94V XL. If they don't fit the Dooper I'll put them back on the Merc 300CE-24 in January. The only concern is that the extra 0.2" of sidewall height might catch on something when the wheels are at full lock.

I have also sourced a skinny spare wheel kit just in case the winter tyre tread pattern and softer compound manages to find a thorn or nail. The emergency repair kit of sealant has in the past never worked and MINI Rescue have said that I would have to wait 3 hours until they could get to me.