How to calculate space saver spare size?

[stephen watson]

Apologies that this is for my other car but I'm hoping people will forgive me - the principal is the same, maybe useful and I'm sure someone on the forum will know the answer:

I'm getting a new eGolf, to compliment my Cali, next week and am thinking of buying a s/h spacesaver wheel/tyre. The eGolf has 16" with 205/55 R16. All the VAG spacesavers on eBay seem to be 16" 125/70.
My understanding is that the 2nd part of the size is the aspect ratio and is the percentage size of the width. So for the eGolf 55% of the 205 width is 112.75 mm - which is the height of the tyre. If you do the same calculation with the space saver 125/70 you get a height of 87.5mm.
This means the overall diameter is smaller so it doesn't seem it is the correct size? Am I right or wrong and if the latter what size do I need to look for please?

 

[sidepod]

I think you may be miss reading. If the aspect ratio is 70 or 80 then it’s prob an 18” rim and thus nearer I the correct rolling radius?

Space saver - load rated?

 

[westfalia]

The diameter difference is about 25mm. If you got a 17" rim would that not make them equal. ?

 

[stephen watson]

I've now found a site that tells me how to do it I think. If you work it out 205/55 16" is the same diameter as 125/70 18". VW has space savers in 16 and 18" wheels, so assuming the most important thing is to get the total diameter the same, counter intuitively (to me any way), it seems if you have a full size 16" wheel you need an 18" space saver. I can find both on Ebay - any further comments?
And by the way, before asking on this forum, I asked the parts department of the VW dealer I'm buying from and they didn't know!