Well, I did the deal on a 530d SE Auto with plenty of options. One of them was the "space saver spare wheel". However, when I checked, the wheel was missing, but the plastic insert containing the jack was in place. It's no problem, as the dealer has promised to put the spare in, prior to me picking it up on Wed.

My question is, is there just one size space saver, regardless of what size wheels are fitted? (this car has 123s which are fitted with 245/40/18 tyres. I would guess so, but just checking. What size tyre is fitted to your space saver wheel?

 

All the same size

 

so those with 19" alloys will have the same space saving spare as 17" or 18" rims. How will it fit? Am I missing something?

 

the spare wheel is only needed for emergency and temporary use... so there will be no high speed driving, all the same size to get you where you need while you fix your wheels... when i use my spare wheel it is actually alot biger than my 17' 122rims...so on the side i put the spare wheel its actually higher than the rest. quite apperent too..

 

The answer lies in the "rolling radius", i.e. the outer diameter of the tyre (that is contact with the road). As the rim size increases, the profile of the tyre decreases. Overall diameter remains the same.

 

It shouldn't be. I think what you are seeing is the optical effect of a 135/80 tyre on a nearly full pressed steel 17" rim against an empty-looking alloy rim on the other wheels. But the radius of the tyre is almost exactly the same:

135 x 80% = 108 mm tyre side height
225 x 50% = 113 mm tyre side height (std tyres)
245 x 45% = 110 mm tyre side height (on 122 rims)

all on 17" rims, so actually your "standard" 225/40 tyre is nearly 5 mm higher per side, and the space saver is the "lowest" of the lot (and, in case you are wondering, it's also slightly smaller diameter than the standard 18" wheel, but only by 6mm overall).

 

I think that the theory is that the space saver is a "get you home measure", and, as has been said, won't be subject to high speeds or loads. The difference in rolling radius is relatively unimportant - differential wear over a short distance is negligible, and LSD's are not needed with DSC. It will screw up the RPA, but you already know you've had a puncture, right?! The ABS sensors won't be affected, but a large difference may "fool" the DSC into thinking that there is slippage on one wheel, but I would imagine that for that to happen, there would have to be a huge difference.
As I'm typing this, I realise that the rolling radius of a flat runflat is probably quite different from a fully inflated one!

For those of us without space saver's who are thinking of getting one, can anyone confirm that the tyres are 135/80 x 17 please?

 

the space saver is 135/80 17.... i think my car was higher because on a normal rim... that ride hight is from the edge(side) of the car.. while on the spare it was very much closer to the center of the car, which may have cause that...
heres a pic...

 

The side position should have no influence - your suspension arm will be at the same height if the wheel radius is the same no matter how wide the wheel is, although again optically it may make a difference as you will see a big gap between the wheelarch and the tyre.

 

The manual clearly says that RPA should be re-initialised when replacing a wheel with the space saver...

The difference in rolling radius IS small (no more than 5 mm with standard tyres/rim combinations; don't know about 19"/20" rims)

Size is 135/80 R 17 on my car.