Mileage Car

Why pressure affects mileage

As a tire rolls, the rubber at the bottom flattens and springs back thousands of times a mile. A soft tire flattens more, and that flexing turns energy into heat. US government guidance estimates mileage drops about 0.2 percent for every 1 psi drop in the average pressure of all four tires, and that keeping tires at the right pressure can improve mileage by up to about 3 percent. Low tires also wear faster at the edges, run hotter and handle worse.

0%Share of mileage lost, on a scale to 40%40%

Find the right number

The correct pressure for your car is on a placard, usually on the driver's door jamb, and in the owner's manual. It's set by the vehicle maker for that car's weight and handling. The number molded into the tire sidewall is the maximum the tire can safely hold. It is not the pressure to use. Some vehicles list different pressures for the front and rear, or for heavy loads.

How to check pressure

  1. Check cold. Measure before driving, or after the car has been parked for a few hours. Driving warms the tires and raises the reading.
  2. Remove the valve cap and press the gauge firmly onto the valve stem until the hissing stops.
  3. Read the gauge and compare it with the placard number.
  4. Add air in short bursts, checking again each time. If you overfill, press the valve core gently to let some out.
  5. Replace the caps and repeat on all four tires. Check the spare too if your car has one.

A simple hand-held gauge is cheap and often more reliable than the gauges built into air pumps.

Pressure and temperature

Air contracts as it cools. A common rule of thumb is that tire pressure drops about 1 psi for every 10 degree Fahrenheit drop in temperature. So tires set correctly on a mild fall afternoon can be several psi low on the first frosty morning. Check more often when the seasons change. Don't let air out of hot tires after a drive, because they'll be low once they cool.

Tire pressure warning lights

Most cars sold in the US since the 2008 model year have a tire pressure monitoring system. It usually warns when a tire is about 25 percent or more below the recommended pressure. That's a safety alarm, set well below the best pressure for mileage. Don't wait for the light. If it comes on, check all four tires soon, and if a tire keeps losing air, have it inspected for a puncture or a leaking valve.

Other tire factors

  • Tire type. Tires differ in rolling resistance. Some are designed to roll more easily. When you replace tires, ask about rolling resistance as well as grip, wear and noise.
  • Alignment. Wheels that are out of alignment scrub sideways as they roll, which costs a little fuel and wears tires unevenly.
  • Winter tires. They often roll with a bit more resistance, but their grip on snow and ice is worth far more. Use what suits your conditions.

Overinflating to chase mileage is a bad trade. It reduces grip and braking performance and can wear the center of the tread. Stick to the placard figure.

Tire pressure FAQ

How often should I check?

Once a month and before long trips is a common habit, and more often when temperatures swing. Tires lose a little air over time even without a puncture.

Does nitrogen help mileage?

Nitrogen leaks out a little more slowly than ordinary air, which can help pressure stay steady. The mileage benefit comes from correct pressure, and regular checks with ordinary air achieve that.