Extreme Terrain surveyed just over a thousand drivers this year and found that roughly one in ten did not know whether their vehicle had a spare at all. That is a fine statistic for people who drive sedans to work.
For anyone who wheels, the more useful finding was the other one: many spares no longer match the tires on the vehicle. That is not a problem of forgetfulness. It is a problem of upgrading, and it is close to universal among Jeep and Bronco owners, because the first thing most of us do is put bigger tires on and leave the factory spare hanging off the back.
Checking a spare takes about five minutes. Here is what to actually look at.
Pressure, which is almost always wrong
A spare loses air like any other tire, except nobody tops it off, so it loses air for years. A spare that has sat untouched since the vehicle was new is frequently flat enough to be useless.
If you carry a full-size spare, which is what a Wrangler, a Gladiator, and a Bronco come with, inflate it to the same pressure you run on the ground. There is no separate spec. It is just a fifth tire.
If you carry a compact or temporary spare, the number is usually much higher. Sixty psi is the common figure, and it is printed on the sidewall. Check the sidewall and your owner's manual rather than assuming, and note the limits stamped alongside it, typically something near 50 mph and 50 miles. Those are not suggestions.
Easiest habit to build: when you check the four on the ground, check the fifth. We covered trail pressures separately in the airing-down guide, but the spare stays at street pressure regardless of what you drop the others to.
How old it is, and how to tell
Tread depth tells you nothing about a spare. A spare that has never touched pavement can have full tread and still be unsafe, because rubber degrades on a shelf the same way it degrades on the road.
Every tire carries a DOT code on the sidewall. The last four digits are the build date: the first two are the week, the last two are the year. A tire ending in 2225 was built in the twenty-second week of 2025. If you find a three-digit code instead of four, the tire was made before 2000 and should be replaced on age alone.
On how old is too old, be careful who you believe. You will see this attributed to NHTSA constantly. NHTSA studied tire aging and issued a consumer advisory in 2008, but it never made a rule, and there is no federal expiration date on a tire. What exists is guidance from vehicle and tire manufacturers, most of whom land somewhere in the six- to ten-year range regardless of tread. Your owner's manual will have a position. That number applies to your vehicle.
While you are down there, look for cracking in the sidewall and between the tread blocks. A spare that lives outside on a rear carrier gets more UV than any tire on the vehicle, which is its own argument for keeping something over it.
The one that catches Jeep owners
This is the part the survey got right and most spare tire advice misses entirely.
Your Wrangler or Gladiator left the factory with a spare that matched. Then you went to 35s. The spare on the back is still the 33 it always was, and it is now roughly an inch smaller in radius than everything else on the vehicle. It looks fine. It bolts on fine. It is not fine.
Put that tire on a 4WD vehicle and the different rolling circumference forces the drivetrain to absorb a difference it was not designed to absorb. On a part-time system, which is what most Wranglers and Gladiators use, there is no center differential to absorb it, so the driveline binds in four-wheel drive. Full-time and automatic systems absorb the difference through a center differential or clutch coupling, and that builds heat.
Here is where we part company with most of the internet. You will see a specific tolerance quoted as gospel: a quarter inch, four thirty-seconds, one percent. Some automakers do publish a limit for their own all-wheel-drive systems, but those figures do not transfer from one vehicle to another. What is genuinely true is that the mechanism is real, that part-time systems and full-time systems tolerate it differently, and that your owner's manual is the only source that applies to your vehicle.
Practically: if your spare no longer matches, treat it as a limp-home tire rather than a spare. Keep speeds down, stay out of 4WD on dry pavement, and get the right tire back on sooner rather than later.
The better fix is to buy five tires instead of four next time. We laid out why that works, and how to rotate the fifth in, in the 5-tire rotation guide. It costs more up front, and it means your spare is always the right size and always the same age as the rest.
If you are not sure what your current setup actually measures, the tire size calculator will convert sidewall sizes into real diameters so you can compare them honestly.
The carrier, the hardware, and the tools
A spare you cannot get off the vehicle is not a spare.
- Break the lug nuts loose once. Carrier studs corrode, and a nut that has not moved in four years can be genuinely stuck. Find that out in your driveway, not on a shoulder.
- Check the carrier mount. On a rear swing-out, look at the pivot and the latch. On an underbody winch carrier, which is what most SUVs and trucks use, make sure the mechanism still turns, and the cable is not seized or rusted through.
- Confirm the tools are still in the vehicle. Jack, lug wrench, and the key for wheel locks if you run them. The wheel lock key is an often-missing item, and without it the spare may as well not exist.
- Make sure the jack can lift your rig. If you have lifted the vehicle or gone up in tire size, the factory jack may no longer have the travel to get a wheel off the ground. We covered that, and why a Hi-Lift is the wrong answer to a flat, in the Jeep flat tire jack guide.
How often to do this
Check the spare's pressure monthly with the other four, and give it a full inspection, covering age, cracking, the carrier, and the tools, twice a year. Tie it to something you already do, like spring and fall oil changes, so it isn't one more thing to remember.
Check it anyway before a long trip, before anything remote, and after you change tire sizes. That last one matters most, because changing tires is the moment your spare silently stops matching, and nothing on the dash will tell you.
If you run a trail checklist, the spare belongs on it. Ours is in the printable pre-trip inspection checklist.
Frequently asked questions
How do I know how old my spare tire is?
Read the DOT code on the sidewall. The last four digits are the build date, with the first two being the week and the last two the year, so a code ending in 2225 means the twenty-second week of 2025. A three-digit code means the tire was built before 2000 and should be replaced on age alone.
What pressure should a spare tire be?
A full-size spare, which is what Jeep Wranglers, Gladiators, and Ford Broncos carry, takes the same pressure as the tires on the ground. A compact or temporary spare usually takes much more, commonly 60 psi, and the correct figure is printed on that tire's sidewall. Check the sidewall and your owner's manual rather than assuming.
Is it bad to drive on a spare that is a different size than my other tires?
On a four-wheel-drive vehicle, it puts strain on the drivetrain. A part-time system, which most Wranglers and Gladiators use, binds in four-wheel drive, and a full-time or automatic system absorbs the difference through a center differential or clutch, which builds heat. The tolerances quoted online, usually a quarter inch or one percent, do not transfer from one vehicle to another, so your owner's manual is the authority. Treat a mismatched spare as a limp-home tire, keep speeds down, stay out of four-wheel drive on dry pavement, and consult your owner's manual.
How often should I check my spare tire?
Check its pressure monthly along with the other four tires, and do the full inspection twice a year, tied to an existing service interval so it is easy to remember. Also check before any long or remote trip, and always after changing tire sizes, since that's when a spare stops matching without warning.
Does a spare tire expire?
There is no legal expiration date on a tire in the United States. NHTSA studied tire aging and issued a consumer advisory in 2008 but never issued a regulation. Most vehicle and tire manufacturers recommend replacement somewhere between six and ten years regardless of tread depth, and your owner's manual will state the position that applies to your vehicle.
Why does my spare tire keep losing air?
Spares lose air at the same slow rate as any other tire, roughly 1 psi a month and more as temperatures drop, but nobody tops them off, so the loss compounds over years. A spare that has not been touched since the vehicle was new is often far below usable pressure. If it is losing air noticeably faster than that, suspect the valve stem or a slow leak at the bead.
Keep the sun off it
A rear-mounted spare takes more UV and weather than any tire on the vehicle, and age and sidewall cracking, not mileage, are what usually retire a spare. A cover is the cheapest thing you can do about that, and it is the one piece of this list that also makes the back of your rig look like something.
Our spare tire covers run 27 inches through 37 inches, with or without a backup camera opening. If you are not sure which size you need, measure the mounted tire with a tape measure rather than relying on the sidewall code, and the sizing guide walks you through the rest.
0 comments