Is Your Spare Tire Flat? Why Spare Tires Often Lose Air Sitting in the Trunk

Many drivers assume their spare tire is ready to go whenever they need it. But one of the most common surprises during a roadside emergency is discovering that the spare tire in the trunk is flat.

Spare tires don’t get used often — and that’s exactly the problem.

Why Spare Tires Lose Air Over Time

Just like regular tires, spare tires naturally lose air pressure. Even when sitting unused in the trunk, they can slowly deflate due to:

• Normal air permeation through the rubber

• Temperature changes (especially cold weather)

• Small valve stem leaks

• Aging rubber

Because they aren’t part of regular tire rotations or inspections, spare tires are often forgotten — sometimes for years.

Cold Weather Makes It Worse

Temperature drops reduce tire pressure. For every 10-degree drop in temperature, tire pressure can fall by about 1 PSI.

That means a spare that was borderline low in the summer may be completely flat during winter — right when you need it most.

The Problem With Discovering It Too Late

Finding out your spare is flat only after installing it can leave you stuck with:

• No usable tire

• No safe way to drive

• Increased risk of rim damage

• A more complicated roadside situation

It turns a simple tire change into a bigger problem.

How Often Should You Check Your Spare?

It’s recommended to check your spare tire pressure:

• Every 3–6 months

• Before long trips

• At the start of winter

• Whenever checking your regular tire pressure

Most compact spare tires (donuts) require higher PSI than standard tires — often around 60 PSI. Check the label on the tire itself.

Don’t Forget About the Tools

Along with checking your spare, make sure:

• The jack is present and functional

• The lug wrench fits your lug nuts

• Wheel lock keys (if equipped) are accessible

• Nothing is missing from the trunk compartment

A spare tire won’t help if the tools aren’t there.

What If Your Spare Is Flat?

If you discover your spare tire is flat when you need it:

• Do not attempt to drive on it

• Avoid driving on the damaged tire

• Call for roadside assistance

In many cases, mobile tire service can:

• Repair the original tire (if safe)

• Inflate and inspect the spare

• Replace the damaged tire on site

• 
  

A Spare Tire Is Only Helpful If It’s Ready

Having a spare tire provides peace of mind — but only if it’s properly inflated and ready to use.

A quick pressure check a few times a year can prevent major frustration later.

If you ever find yourself with a flat tire and a flat spare, mobile tire service can help get you back on the road safely.