Yes, once it's more than a few psi down. If the TPMS light is on, the tire is at least 25% below the pressure on your door sticker, and that's low enough to overheat the tire, wear the edges and add stopping distance. Drive gently to the nearest air pump today, not next week.
A tire that's visibly flat is a different story. Don't drive on it at all; fit the spare.
What low pressure does to the tire
Air carries the load, not rubber. Take the air out and the sidewall flexes more with every rotation, which builds heat. Heat is what kills tires.
NHTSA's TPMS rule lists the failure modes: skidding and loss of control, hydroplaning, longer stopping distances, flats and blowouts, and overloading. That's why the rule exists. NHTSA expected the TPMS requirement to prevent about 119 to 121 deaths a year once every car had it.
The handling loss is measurable. Tire Rack ran a car at 30% under pressure, 29 psi down to 20 in front and 33 to 23 in back. The underinflated set was over 2 seconds slower around their handling course, about a 7% loss, and the car needed more steering input and reacted later. Their conclusion was that you can't use your eyes as a pressure gauge, because a modern low-profile tire looks fine at 20 psi.
There's a wallet cost too. Fueleconomy.gov puts the fuel penalty at about 0.2% for every 1 psi the average pressure drops. NHTSA says properly inflated tires can add about 4,700 miles of tread life and save as much as 11 cents a gallon.
Why it's low, in order of likelihood
Cold weather first, then slow leaks, then a nail or a bad valve.
Goodyear's figure is 1 to 2 psi lost for every 10°F drop in temperature. A 32 psi tire set in a 70°F garage reads about 28 psi on a 30°F morning, and that's enough to light the TPMS on some cars with no leak at all. Tires also seep air slowly over time, which is why every tire maker says to check monthly.
If one tire is lower than the other three, it's a leak. Nails, a corroded wheel bead, a cracked valve stem, or a bent rim from a pothole are the usual suspects. If all four are down together, it's weather or neglect.
How far can you drive, by how low it is
Use the placard pressure, not the number on the tire sidewall. The sidewall number is the maximum, not the target.
How low | Example on a 35 psi placard | What to do | How far |
|---|---|---|---|
1–4 psi under | 31–34 psi | Top up at your next stop | Normal driving for a few days |
5–8 psi under (TPMS may light at 25%) | 27–30 psi | Fill today, drive gently | A short local trip to a pump |
9–15 psi under | 20–26 psi | Fill now, check for a leak | A mile or two at low speed |
Visibly flat or under 15 psi | Below 20 psi | Fit the spare or call a tow | Zero |
The TPMS threshold comes from the federal standard: the light must come on when any tire is 25% or more below the recommended pressure. On a 35 psi placard that's about 26 psi. On a 32 psi placard it's 24. So a lit light doesn't mean "slightly low," it means you've already lost a quarter of the air.
Skip the highway until it's filled. Speed is heat, and heat on a low tire is how you get a blowout.
Checking it properly
Cold. Bridgestone defines cold as parked for three hours or more, or driven less than a mile. Driving warms the air and raises the reading by a few psi, so a tire checked after a commute looks healthier than it is.
The right number is on the Tire and Loading Information label. NHTSA says it's on the driver's door edge or the B-pillar, and in the owner's manual. Many cars list different front and rear pressures, so read both.
A cheap pencil or dial gauge in the glovebox beats the gas station one, which gets dropped a lot. Most tire shops will check and fill for free if you ask.
Check the spare too. Nobody does, and a compact spare at 20 psi when you need it at 60 is useless.
When to stop and fit the spare
If the tire is flat enough that the sidewall is folding under the rim, stop. Driving on it crushes the inner liner and turns a $30 repair into a new tire. Same if you can hear air escaping, or the car pulls hard to one side.
A TPMS light that comes on and then goes off a mile later is usually just cold air warming up. One that stays on, or comes back the next day, is a leak. Find it this week.
What to do today
- Find the placard on the driver's door jamb and note the front and rear pressures.
- Check all four tires cold with your own gauge, before driving if you can. Write the numbers down.
- Fill any low tire to the placard figure. If it's more than about 10 psi down, drive to the pump slowly and keep it off the highway.
- Recheck tomorrow morning. A tire that dropped more than 2 psi overnight has a leak; look for a nail and book a repair.
- Reset the TPMS if your car needs it (check the manual) and check the spare while the gauge is out.
Sources
- NHTSA: Tires
- Federal Register: FMVSS 138 Tire Pressure Monitoring Systems final rule
- Fueleconomy.gov: Keeping Your Vehicle in Shape
- Tire Rack: Air Pressure, Correct, Underinflated and Overinflated
- Bridgestone: How to Check Tire Pressure
- CCJ: Monitor Tire Pressures as Winter Temps Drop (Goodyear)
Featured image: Close-up of air pump pressure gauge and a car tire in the blurry background by Ivan Radic, CC BY 2.0. Source
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