My 2010Volvo XC70 said rear fitment with or without trailer. But the OD of the tyre was 2 inches less than the standard OEM alloys fitted so would have imbalanced the steering if fitted to the front. The space saver was load rated for the cars GVW and as far as it would be aware the nose load was no different to an extra load in the luggage bay.
I can't imagine i would start rotating tyres, in the rain, stuck in some layby!
The sad part is that some people cannot afford to pay to have the tyre replaced and will carry on using the space saver.
Mine gives little information.
A bit of a blow if towing a caravan. With our Volvo S80 which came with a spacesaver I bought a full size wheel and it only just fitted into the well however raised the floor by about 2 inches.
If it’s some types of 4WD then a minimum of two tyres may require to be changed depending on the tread depth difference between the tyres and the new one. Haldex seem reasonably tolerant, Subaru specify 1 mm overall difference between all four tyres. Luckily I converted my previous Subaru to full size spare and when I had a non repairable puncture the spare which was a part worn was okay to be matched with the other three tyres. So I only had to buy a new full size spare, keeping fingers crossed I did not have another non reparable puncture.
My Skoda Superb had a full diameter and width tyre but on a 16 inch rim. Whereas the OEM alloys were 17 inch rims. So the spare was still labelled 50 mph but given it was in all respects a standard rolling diameter and width I would have been happy to travel long distances on it if required.
I hardly think that a car manufacturer would provide a space saver if it were likely to burn up a differential. After all he is responsible for his own product liability so he would have an obligation to inform the consumer under what conditions a space saver may cause damage to components of the transmission.
Surely then the same theory holds good even when not towing.
A differential will cope just fine with the speed/distance limits imposed on the use of a space-saver. Older 4x4 systems with no centre differential would suffer from wind-up in normal use.
Towing will put more load on the differential, meaning higher operating temperature, meaning lower oil viscosity, meaning thinner oil film thickness, meaning possible reduced protection. Under these conditions, if the differential is also being asked to manage a constant speed difference between the axles then this is an additional load factor.
Space saver wheels usually come with a maximum speed and/or distance warning (often presented on the wheel itself, as well as in the user manual), no doubt at least in part to minimise the impact of wheel speed difference.
If such conditions were to lead to possible premature failure of vehicle components then it's the manufacturer's responsibility to issue an appropriate warning.
The restrictions applicable to space savers are based on the limited durability and less favourable vehicle handling characteristics of the much slimmer space saver compared with those of a full size wheel. They therefore apply equally to vehicles with two and four wheel drive.
Given the combinations of transmission systems the AA advice isn’t particularly helpful for towing. But at least it refers to the Owners Manual, which in some cases is devoid of information.
They do (as I outlined).
As I said, the differential issue will be one of the factors influencing the warning - others are also important (as you describe). All cars have some kind of differential which will work harder if mismatched wheels are fitted. Keeping the speed down (as per the space saver warning) will mitigate this and other risks.
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