Found this on space savers:
What are they: These are much thinner than standard car tyres and can be fitted to a vehicle in the event of a puncture.
Are they safe to use: Yes. Provided drivers are aware of the restrictions that apply to their use. Users must remember space saver wheels are intended to get the vehicle to a place of safety and not day-to-day motoring.
Is there a maximum speed for space saver wheels: The maximum speed restriction for a space saver wheel is likely to be 50mph.
Exceeding this is dangerous and could result in prosecution. The wheel itself should have a label or moulding that carries its speed restriction.
Is there a maximum distance for space saver wheels: There is no official restriction on how far a driver can travel on a space saver wheel, but around 50 miles is considered to be the maximum safe distance.
The tyre will have a shallow tread depth, compared with standard tyres, and this will degrade at a faster rate due to the narrow gauge of the wheel. Using a space saver wheel will reduce the car’s cornering grip by 10-15 per cent.
Tread depth rules apply to space saver wheels in the same way as they do to standard tyres.
Can space saver wheels be used front and back: Check your handbook, because some vehicles have restrictions on where they can be used for safety reasons. Never use more than one space saver wheel on a vehicle at any one time.
Will there be any difference in how the car drives: Yes. It’s likely the car’s handling will be changed by the non-standard wheel. This will reduce the vehicle’s cornering and braking abilities.
Can space saver wheels be used on motorways: It’s not illegal, but driving at 50mph is dangerous on motorways. HGVs will be travelling at 56mph, so a car moving at just 50mph in the left-hand lane is likely to cause a dangerous obstruction. Avoid motorways if at all possible.
Remember to check the space saver’s air pressure: The majority of space saver wheels have pneumatic tyres, so you’ll need to periodically check your ‘spare’ is at the correct pressure. Driving on an under-inflated space saver would be extremely dangerous.
Can I use a space saver wheel for my MOT: Not unless you want to fail. Wheels of the same ratio must be fitted to each axle.
VERDICT: Changing a wheel has become a bit of a dying art, but for those who are confident with a jack and brace, the space saver offers a virtually guaranteed roadside get out of jail card for anyone who’s suffered a puncture or blowout.