Estimate savings from correct tire pressure.
Under-inflated tyres flex more, and that flexing is energy converted to heat rather than motion. The Department of Energy figure is roughly 0.2% of fuel economy lost per psi below the placard pressure across all four tyres, so 6 psi low costs about 1.2% — modest in isolation but permanent, and it compounds with a tyre life penalty and worse wet braking. The saving here is calculated on gallons rather than by applying the percentage to your fuel bill, which is the correct order for an mpg change. The real argument for checking pressures monthly is not the fuel: tyres lose about 1 psi a month naturally and more in cold weather, and a badly under-inflated tyre can fail.
Tire Pressure Fuel Savings
Economy loss% = psi below placard x 0.2; under-inflated mpg = correct mpg x (1 - loss%); saving = (miles/under-inflated - miles/correct) x fuel price
Correct tyre pressure is a safety matter as well as an economy one. Under-inflation increases the risk of heat build-up and tyre failure — always set pressures to the vehicle manufacturer's placard specification when the tyres are cold.
The figure on the door jamb or fuel flap placard, which is the vehicle manufacturer's specification. The number moulded into the sidewall is the tyre's maximum, not a recommendation.
It gains very little and costs grip, ride quality and even tyre wear across the tread. A few psi above placard is sometimes used for heavy loads; well above it is a safety trade for a negligible saving.