I have 18" wheels and I want to get a set of 16" or 17" for offroad/winter. The problem is that I have a full-size spare and that is 18".

So, if I swap the wheels to 17" can I keep the 18" spare or do I need a 17" spare also?

Basically, should I be looking to buy 5 17" wheels or can I get away with 4?

You should be OK because the rolling radius of the 17 and 18 is the same due to the change of side wall profile between the two tyres.

Plus as its a spare it wont be used for a long time.

Not sure it's that easy. If your spare is a proper steel one with yellow 50mph sticker then you're fine. If it's a normal roadwheel you will fall foul of the regs that say all tyres on axle must be same size.

That is what I thought and why I asked... thanks...

It's 5 then

Of course you can get away with four IF the spare is going to be used as a spare, as in the case of a puncture and not used in normal use. Why else do manufactures supply a skinny tyre for a spare ? because that is what it is for, to get you home or to a garage.

It is only good practice to have the same tyres on an axle, not a legal requirement and as said, as long as the rolling radius is the same then no damage to the diff will occur.

You will have the same rolling radius with the 17" and 18" so it will be fine as a 'get you home' tyre.

Understood. That is good news for my wallet!

I've had a look around and I'm gobsmacked. There don't seem to be any regs about this. I stand corrected (again!). That said, I wouldn't mix different sizes myself, even if the circumference is the same.

I am rather gobsmacked too!

As far as I can tell the only rule I can find says you cannot mix radial and crossply on the same axle.

Tyres come under Road Vehicles (Construction and Use) Regs 1986 as amended and Section 40 of the Road Traffic Act 1988 as amended.

A jargon free concise synopsis is below.

UK Tyre Law
For a tyre to be deemed legal for road use, there are certain legal requirements that must be complied with. There are two aspects of UK tyre law:

Tyre manufacturers must comply to certain regulations if their tyres are to be considered legal. These include sidewall markings and tyre construction.
The driver of a vehicle is responsible for the condition of the tyres on the vehicle they are driving. This includes tread depth, tyre condition and that the tyre is fitted correctly.
The penalties if you are found to be using a vehicle with illegal tyres are currently a fine of up to £2500 and three penalty points for EACH tyre. Correct as of Dec 2006

Tyre Regulations
The two legal statutes that relate to tyres are:

The Road Vehicles (Construction and Use) Regulations 1986
This does not only relate to tyres, but all aspects of road usage. The parts of these rules that concern tyres cover the use of temporary and space saving spare wheels, tyre loads and speed ratings, the requirements of tyre mixing, and tyre condition and maintenance.

Motor Vehicle Tyres (Safety) Regulations 1994
This is more specific to tyres. It covers the requirements relating to the supply of tyres, their "e" markings as well as covering the regulations for part-worn and re-tread tyres.

Temporary Tyres and Space Saving Spare Wheels covered by Regulation #24
Space saving spare wheels and any other tyres marked "temporary use only" are restricted to 50mph and should be changed at the earliest opportunity.

Tyre Loads and Speed Ratings covered by Regulation #25
All tyres must have a service description which describes its speed rating and load index.

It must be ensured that the tyres fitted to a vehicle are capable of operating to the loads and speeds that that vehicle will be subjecting the tyres to. The driver of a vehicle should check to see that the tyres are capable of carrying the loads and dealing with the speeds that the vehicle will be doing. If the tyres are found to be unsuitable, then the driver is responsible.

Tyre Mixing covered by Regulation #26
Tyre mixing is the use of tyres of a different construction on the same vehicle. Tyres can be either "Cross Ply" or "Radial" and although tyre mixing is more pertinent to commercial vehicles, it does apply to cars and light vans too.

It is illegal to have radial tyres on the front wheels with cross ply tyres on the rear and it is also illegal to have one type of tyre on one side of a vehicle with the other type on the opposite side.

Articulated vehicles are treated as two vehicles.

Tyre Condition and Maintenance covered by Regulation #27
Tyre Condition
The law states "…the tyre has any lump, bulge or tear caused by separation or partial failure of its structure" then it is illegal. When assessing damage caused to a tyre, it is always worth having the tyre removed from the rim and inspected both internally and externally by a qualified fitter.

If a tyre has a cut that is deep enough to reach the cord or ply and is more than 25mm or 10% of its width (whichever is the greater), then that tyre is considered illegal. It is also the case that if any cut, however small, exposes the cord of a tyre, then the tyre is considered illegal.

Tyre Pressure
The law states that a tyre must be "inflated as to make it fit for the use to which the motor vehicle or trailer is put". This is quite ambiguous as there is no indication as to when a tyre becomes too over or under inflated to perform its purpose safely.

The correct pressure to inflate your vehicle's tyres to is the one recommended by the vehicle manufacturer, not the maximum pressure found on the tyre's sidewall.

The correct pressure can be found in the vehicle's owner manual and on later vehicles (2003 onwards) on the vehicles tyre information placard, usually found on the b-pillar, which is at the rear of the driver's door.

Tread Depth
Tread depth is a major factor in whether a tyre is illegal or not.

The minimum tread depth for cars and light trailers (which includes caravans) of up to 3500kgs or 8 passengers (including driver) is a minimum of 1.6mm in a continuous band across the central three–quarters of the tread width of the tyre and throughout the whole of its circumference.

If the gross vehicle or train weight is above 3500kgs or if the vehicle is a motorcycle above 50cc, the law is different. The grooves of the tread must be a minimum of 1mm in a continuous band across the central three–quarters of the tread width of the tyre and throughout the whole of its circumference.

If the grooves of the original tread pattern do not extend over three–quarters of the width of the tread, which is quite common with motorcycle tyres, any groove that is a part of the original pattern must have a minimum of 1mm tread depth

PaulC wrote:

I have 18" wheels and I want to get a set of 16" or 17" for offroad/winter. The problem is that I have a full-size spare and that is 18".

So, if I swap the wheels to 17" can I keep the 18" spare or do I need a 17" spare also?

Basically, should I be looking to buy 5 17" wheels or can I get away with 4?

No I won't. I always carry a spare and know how to fit it.