Your Tire Light Doesn’t Come On Until 25% Low
Just the Tip:
By the time your tire-pressure warning light comes on, a tire can be 25% low. Well before that point, an underinflated tire is less safe and wears out faster. Check every tire with a gauge once a month, and fill each to your car’s recommended pressure, not the maximum on the sidewall.
Federal safety standard FMVSS 138 requires the warning to come on within 20 minutes once a tire falls 25% below the pressure on the car’s tire-pressure label. On a car that calls for 35 psi, a tire can sit near 26 psi with the dashboard still dark.
That makes the light a safety floor, not a maintenance reminder. Fuel economy slips long before it comes on. By the figures on fueleconomy.gov, the Energy Department and EPA site, mileage falls about 0.2% for each psi the four tires average below where they should be. Keeping them at the right pressure improves mileage 0.6% on average, and up to 3% in some cases, the site says.
The money is real but modest. With regular gas at $4.48 a gallon in the Energy Information Administration’s September 21 survey, a car driven 12,000 miles a year at 25 mpg burns about $2,150 in fuel. By that math, the typical 0.6% gain is worth about $13 a year, and the 3% best case about $65. The bigger reasons are safety and wear, since the same site notes that tires kept at the right pressure are safer and wear out more slowly.
So check them yourself once a month, along with the spare, before the car has been driven or after it has sat for three hours. Use the pressure on that label, usually in the driver’s door jamb, or in the owner’s manual, never the maximum molded into the tire’s sidewall, and add air until each tire matches it.
One related myth is worth dropping. Replacing a clogged air filter doesn’t improve fuel economy on fuel-injected, computer-controlled engines, which covers cars built since the early 1980s. It can improve acceleration.
Keep the gauge in the glove box and tie the check to something you already do monthly, like paying a bill.
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