MOT testing a car on a spare tyre

I could be totally wrong, but...

space saver wheels and tyres are temporary use only. During use on the road a speed restricted wheel such as a space saver will not perform well and safe enough over long road journeys, the ESP, ABS, steering can all become affected by the smaller size wheel and tyre fitted, and that does not include the possibility of the tyre overheating.

A full size temporary use wheel can be manufactured cheaper so that it is cost effective for the manufacturer. The combination of the full size wheel and tyre should have no adverse effects on the above areas I mentioned, thus road safety is not compromised. The disadvantage is that the manufacturer either has to build a large enough boot well to accommodate the wheel and tyre, or put a tyre cage or support underneath the vehicle, again at a cost.  

There could be many other factors to consider over and above anything here, and of course I could be completely wrong above.

Would a space saver with same size tyre have a Tpms valve fitted ?

most of the ones ive seen?no.not rqrd cos it needs fixing and you have a bright yellow light on the dash as well now so pretending you forgot dont fly.if what you mean is now we fail it on tpms on instead youve gotta prove the systems faulty rather than indicating a pressure fault(which it will till fixed and reinstalled so not a system fault now is it) and you have to manage that without dismantling/plugging in so 99% of the time its p+a anyway with the added bonus that theyve taken the a away so you have to type it out yourself.yes i know that looking at the valves will give you a pretty good idea,i know about(and use)the remote control jobby that reads the info and will tell you the data from the wheel sensors so that you will KNOW its fooked but none of any of that is part of the test/approved equipment so it dont count now does it.typing/advising wheel pressure sensors inop,unable to fail,refer to dvsa dont get you any friends either but i didnt write the book now did i.....

It’s got very little to do with ‘space saving’ and not a great deal about cost saving either actually. It’s mostly about weight saving and thus bringing the official emissions figure down.

Agreed, there is no failure for matching sizes across an axle, neither is there a failure for steel versus alloy wheel across an axle provided the tyres match.

Maybe the point has been missed. Putting aside skinny space savers, lets focus on full sized ones, with a matching tyre size.

There must be a reason for the restricted speed warnings. If the sizes match, and also load index etc as in the FB example, there must be other factors in the tyre construction that require the warning labels from the manufacturer.

Lets take the tread depth as an example & look at how so many testers struggle with primary grooves, TWI and secondary grooves, and the 1mm rule.

Without having opportunity to have a look at a full sized space saver, my point is are there other sidewall markings that testers may be overlooking in making a pass/fail decision.

I'm thinking more along the lines of the examples of NHS & "E" markings & asking is this something a tester could be missing due to a lack of understanding along the same lines as the tread depth example.

Is that different to what I said?

Could it be the steel wheel is a different width to the alloy thus making the vehicle a bit less stable, or thinking back to some of the Renaults where if you fitted the steel spare you had to use different nuts as the taper wasn't the same, maybe using the same nuts  renders the vehicle not so safe hence the speed restriction.

You have no idea of the origin of that wheel/tyre. It’s most likely that the rim is as standard BUT the majority of the range are fitted with upgraded wheels, alloys in a larger size most likely. Where that is the case, the spare would just be a standard rim, manufacturers aren’t going to peel the stickers off because a base model is fitted with the same all round but the other 5 trim levels are on different alloys.

It’s really only difficult if you want it to be, the rim is the same as any other, as is the tyre. It’s really that simple.