Pump up tires to the factory-defined value. Pumped tires increase rolling resistance point and therefore fuel consumption.
There are also tires designed to save fuel (with less resistance). Do not drive with winter tires in summer and vice versa. Summer tires are made for summer and winter for the winter.
If you do the opposite, the vehicle will consume more fuel, your tires will dissipate quickly, and most importantly is your safety on other road users. Tires lose air due to time (about 1 psi per month) and temperature (1 psi for every 10 degree drop); under-inflated tires have more rolling resistance, which means you need to burn more gas to keep your car moving.
Buy a reliable tire gauge and check your tires at least once a month. Be sure to check them when they are cold, since driving the car warms up the tires along with the air inside them, which increases pressure and gives a falsely high reading. Use the inflation pressures shown in the car owner's manual or on the data plate in the driver's door jamb.
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