My Car Has TPMS. Do I Still Need a Tire Gauge?

The myth: "The car will tell me"

It's a fair assumption. Every new car and light truck built since September 2007 has a tire pressure monitoring system, and a light on the dash that should come on when a tire is low. So why would you still keep a cheap gauge in the glovebox?

Because of what "low" means to the system. TPMS was designed to catch a tire that has lost a lot of air, not to keep your pressure right. Those are different jobs, and the second one still belongs to a gauge.

What the federal rule actually requires

TPMS comes from the TREAD Act. NHTSA's standard, FMVSS No. 138, phased it in so that all light vehicles built on or after September 1, 2007 have it. The core requirement:

• The warning must come on when a tire is 25 percent or more below the vehicle maker's recommended cold pressure (the number on your door placard).
• It must come on after no more than 20 minutes of driving at normal road speeds.
• A separate malfunction signal has to tell you when the system itself isn't working.

Automakers can set their systems to warn sooner, but 25 percent is the line the law draws. The same standard requires owner's manuals to say that TPMS is not a substitute for proper tire maintenance, and that keeping the correct pressure is the driver's responsibility even when the warning hasn't come on. NHTSA's tire page puts it plainly too: these systems only warn when a tire is significantly underinflated. Here's what that looks like on a few common placard numbers:

A tire 17 or 19 percent low is a tire that's working too hard, running hotter and wearing faster, and on many cars the dash will say nothing about it.

What NHTSA found when it checked real cars

In 2010 and 2011, NHTSA surveyed vehicles at gas stations across the country, measuring the actual pressure in all four tires. Its report, DOT HS 811 681, is the best evidence on how well TPMS works. The findings cut both ways:

The good news. Among 2004–2007 model-year vehicles, 23.1 percent without TPMS had at least one severely underinflated tire (25 percent or more low). With TPMS, that fell to 11.8 percent, and to 5.7 percent for 2008–2011 models with TPMS. NHTSA estimated TPMS produced a 55.6 percent reduction in the likelihood of a vehicle having a severely underinflated tire.

The part the myth misses. NHTSA found TPMS was also effective, over 35 percent, against moderate underinflation of 10 percent or more. But in its words, it does not appear to have an effect on mild underinflation. And across all 2004–2011 passenger vehicles, 12.4 percent still had at least one severely underinflated tire.

So TPMS does its job: it catches big losses. Keeping all four tires at the right pressure day to day is still on you.

Direct vs indirect: some systems can't read pressure at all

NHTSA's report describes two kinds of TPMS:

• Direct systems put a pressure sensor inside each wheel, usually on the valve stem, with a battery-powered transmitter. Some show each tire's pressure on the dash. NHTSA found cars with tire-specific pressure displays had the lowest rate of severe underinflation in the survey.
• Indirect systems don't measure pressure. They use the anti-lock brake wheel-speed sensors and look for a wheel turning faster than the others, a sign its tire is smaller and may be low. NHTSA notes early indirect systems had trouble spotting low tires when several were low at once.

Indirect systems were rare by the time of the survey, fewer than 3 percent of the vehicles observed, but if your car has one, there's no number anywhere on the dash. The gauge is the only way to know the pressure.

Even a pressure readout isn't the whole story

If your dash shows each tire's psi, that's useful. Two things to keep in mind:

1. Temperature changes the number. USTMA says pressure rises or falls about 1 to 2 psi for every 10°F change. After a drive, or a car parked in the Florida sun, the reading will be higher than the cold pressure the placard refers to. Set pressure cold, meaning parked for at least three hours.
2. The spare. NHTSA's advice is to check all tires, including the spare. A flat spare is worth knowing about before you need it.

Our pressure and temperature tool shows how much a reading moves with heat.

Why the small losses matter

A few psi doesn't sound like much, but it adds up:

• Fuel economy. fueleconomy.gov says underinflated tires can lower gas mileage by about 0.2 percent for every 1 psi drop in the average pressure of all tires, and that proper inflation improves mileage 0.6 percent on average, up to 3 percent in some cases.
• Wear and heat. NHTSA's report notes underinflated tires flex more, which brings longer stopping distances, sluggish handling and heat buildup.
• The slow leak you'd otherwise miss. A tire that's always a few psi lower than the others is often a nail or a valve. A gauge spots the pattern long before the light does.

The routine that works with TPMS

Think of the TPMS light as a smoke alarm and the gauge as checking the stove. You want both.

1. Keep a gauge in the car. NHTSA recommends it. A simple dial or digital gauge works; our guide to checking tire pressure compares the types.
2. Check monthly, cold, against the door placard, not the sidewall maximum. Our post on why not to inflate to the sidewall number explains the difference.
3. Treat the light as urgent when it does come on. Our TPMS light guide explains solid vs flashing.

If the light flashes at startup and then stays on, the system has a fault: often a sensor battery or a sensor damaged during a tire change. Our TPMS service diagnoses and replaces sensors. In Miami-Dade, Broward and Palm Beach we do it at our Sunrise shop or with our mobile van. If a tire keeps losing air or is worn out, shop by size or vehicle; shipping is free to the 48 contiguous states and DC.