Written by the fitting team at 24/7 Mobile Tyre Services, Glasgow. Last reviewed: July 2026.
Boot open. Wheel brace in one hand. Spare in the other, and it is soft.
Before anything else: virtually nobody checks their spare. It is the only tyre on the car with no warning light, no visible wear and no reason to ever think about it. You are not the exception here, you are the rule.
Right. What can still be done.
Quick answer: If both the road tyre and the spare are flat, your options are to inflate the spare with a 12V compressor if it still holds air, call a mobile tyre fitter to come to the vehicle, or arrange recovery. Do not drive on a flat or badly under inflated spare. A space saver typically needs far higher pressure than a normal tyre.
First: is the spare flat, or just under inflated?
This distinction decides everything, and it turns on a fact almost no driver knows.
A space saver usually runs at a much higher pressure than a road tyre, commonly in line with manufacturer guidance on space saver spares — often around 60 PSI, or about 4.2 bar.
Read that again if you have been running road tyre pressures in your head. Check the label on the spare wheel itself, the boot lid sticker, or the handbook, because space saver pressures do vary.
Here is why it matters so much. A space saver sitting at 30 PSI looks completely normal in the boot. It is firm to the touch. It is also nowhere near usable.
That one fact explains a very large share of “my spare was flat too” calls. The spare was never punctured. It was simply never inflated to its own specification, and it has been quietly losing air for six years.
Full size spares run at the normal road pressure for that wheel position.
The test:
- It holds air → it is under inflated, not punctured. A compressor fixes it in three minutes
- It takes air and immediately loses it → punctured or perished, and it is not going on the car
One caveat. The squeeze test tells you nothing at these pressures. A 60 PSI tyre and a 35 PSI tyre both feel hard to a thumb. Use a gauge.
Your four options right now
1. Inflate the spare, if you have a compressor.
A 12V compressor running off the accessory socket will inflate a soft space saver in a few minutes.
Two practical notes. Many cheap compressors struggle above 50 PSI and take a while getting there. And they get hot, so let it rest if it needs to.
If it inflates and holds, fit it and go, at reduced speed.
2. Sealant kit, but only sometimes.
It may seal a small puncture in the tread of the road tyre, though as TyreSafe’s guidance on spare wheels and inflation kits notes, it will do nothing at all for a sidewall split, a large hole, or a tyre that has come off the rim.
Check the expiry date on the canister. It is printed on the bottle, and a great many sealant kits in British boots are years out of date.
Know the trade off too. Sealant fouls the TPMS sensor and makes the eventual repair messier and dearer. Tell your fitter you have used it.
3. Mobile tyre fitter to the vehicle.
The tyre is brought to the car, fitted, balanced and torqued where it stands. No recovery, no towing, no garage day.
4. Recovery.
Breakdown cover or a paid flatbed to a garage. The honest downside: it is a tow to a garage that may well be shut, and your car ends up somewhere you are not.
Framing the choice properly. If it is the middle of the night, a Sunday or a bank holiday, options 3 and 4 are the only live ones. Only one of them ends with you driving your own car home tonight.
What NOT to do
- Do not drive on a flat. The tyre comes off the bead, the rim runs on the road, and a tyre bill becomes a wheel bill. See what to do when a pothole buckles an alloy wheel.
- Do not drive on a seriously under inflated space saver. They are narrow, they carry a substantial share of the load, and under inflated they overheat fast. “Just to the garage” is exactly the distance that destroys them.
- Do not exceed the space saver’s speed limit, commonly 50 mph, or its distance limit. Handbook rules apply.
- Do not fit a space saver to a driven axle if your handbook says not to. On some cars the correct move is to swap a full size wheel across from the non driven axle instead. Check before you improvise.
- Do not run two space savers at once. They are designed for one wheel position, temporarily.
- Do not attempt a wheel change on a motorway hard shoulder or in a live lane. See a flat tyre on a smart motorway, and the Highway Code’s rules for breakdowns and incidents.
- Do not try to inflate a tyre that has come off the rim with a small compressor. It will not seat, and forcing it is genuinely dangerous. More on that in alloy wheels leaking around the bead.
Why your spare was flat, and it is probably not what you think
It has never been checked. Servicing rarely includes the spare as standard unless you ask. The MOT does not test it. It has no warning light. It quietly loses a couple of PSI a month for six years and nobody notices.
Permeation. Every tyre loses air slowly through the rubber. Road tyres get topped up. Spares do not.
The valve is old. A valve that has not moved in a decade cracks at the base.
It has aged. Full tread, no wear, and rubber that has been sitting in a damp boot well since the car was new.
Check the DOT code on the sidewall. A ten year old spare with perfect tread is still a ten year old tyre. We covered that properly in old tyres with good tread.
It was never right. Cars bought second hand often come with a spare that was already soft. Nobody checks at handover, on either side.
Salt and damp in the boot well. Corrosion on a steel spare’s rim causes exactly the bead leak that costs it air.
While you are here, check the rest of the kit
You are already at the boot. Turn a bad day into a useful one.
- ☐ Is there a spare at all? Many cars ship with a sealant kit only. Lift the boot floor and look. Do it now, not on a hard shoulder in December
- ☐ The jack. Present, complete, and does the handle exist?
- ☐ The wheel brace. Present, and does it actually fit your nuts?
- ☐ The locking wheel nut key. The most commonly missing item in any British boot. See what to do when you cannot find it
- ☐ The sealant canister expiry date
- ☐ A 12V compressor and a decent pressure gauge
- ☐ Gloves, a torch and a hi vis vest. A hi vis is legally required in the boot in several European countries and is plain common sense on an unlit Scottish road
On the compressor. A basic 12V unit is probably the best twenty five pounds a driver can spend. It turns “my spare is flat too” from a recovery job into a five minute inconvenience.
It also lets you check the spare without dragging it out of the boot well, which is why people actually do it.
Recommending that costs us callouts. It is still the right advice.
The five minute check that prevents all of this
- Check the spare’s pressure whenever you check the others. Realistically every couple of months, and always before a long trip or a ferry
- Set it to its own specified pressure, not the road tyres’ figure
- Check the DOT code once a year
- Have the spare inspected at each service or tyre change. It is the one time the boot floor is up anyway
- Do it in late autumn. Pressures fall in the cold, so an October check catches the spare before winter does. More on that in why a tyre keeps losing air
- For caravans, motorhomes and trailers the spare matters even more and ages faster. See a caravan tyre failing on the A82
The pattern we see from the van
When somebody rings and says “I’ve got a flat,” our second question is almost always about the spare. And a surprising share of the time the answer, after a pause, is “actually, hang on.”
There is one scenario that arrives like clockwork. The end of school holidays, the airport long stay car park, late evening. Two weeks of parking, one soft tyre, luggage on the tarmac and children who are past their limit.
Nobody checks their spare before a holiday. Everybody wishes they had at 11pm in a car park.
FAQs
What pressure should a space saver spare be? Usually much higher than a road tyre, commonly around 60 PSI, but it varies by vehicle. Check the label on the spare wheel itself, the boot lid sticker or your handbook rather than assuming.
How often should I check my spare tyre? Every couple of months, whenever you check the others, and always before a long trip or a ferry crossing.
Can I drive on a slightly flat spare? No. A space saver is narrow and carries a large share of the load. Under inflated, it overheats quickly and can fail. Inflate it to its specified pressure or do not use it.
How fast can you drive on a space saver? Commonly limited to 50 mph, and the limit is usually printed on the wheel itself. Check your handbook, because limits vary.
How far can you drive on a space saver? They are designed for limited distance, and manufacturers state a figure in the handbook. Treat it as getting you to a fitter, not as a temporary tyre.
Do spare tyres expire? They age like any other tyre, regardless of tread. Rubber hardens and can crack even sitting in a boot. Check the DOT date code, and treat a spare over ten years old as due for replacement.
Does the MOT check the spare wheel? No. A Class 4 car MOT does not test the spare. That is a big part of why so many spares are flat.
Can a mobile fitter come out if both my tyres are flat? Yes. That is one of our most common callouts. Give us the tyre size from the sidewall and your location, and we will bring the tyre to the car.
If both are out of action, the car does not need to move
There is nothing left for you to try, and that is fine.
Tell us the tyre size from the sidewall and where you are. We will come to it. Airport long stay car parks, driveways, roadside, any hour of any day.
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Related: emergency tyre replacement, mobile tyre fitting in Glasgow, puncture repair, locking wheel nut removal, TPMS sensor replacement. Also: what to do when you cannot get to a garage.
