Tire Pressure Adjustment Calculator

Tire pressure moves with the weather, and it moves more than most drivers expect. The correct pressure is not the maximum printed on the tire sidewall. It is the value on the door-jamb sticker, and that value is specified as a cold pressure, meaning the tire has been parked for at least three hours or driven under a mile.

The formula is the gas law, not a rule of thumb

A tire is a sealed container of roughly fixed volume, so the pressure inside tracks absolute temperature:

P₂ = (P₁ + 14.7) × (T₂ / T₁) - 14.7

Everything in that expression is absolute. Gauge pressure becomes absolute by adding the 14.7 PSI of atmosphere pushing back on the outside of the gauge, and Fahrenheit becomes Rankine by adding 459.67. Subtracting the 14.7 at the end turns the answer back into what your gauge will actually read.

Why the popular rule is “0.1 to 0.2 PSI per °F”

Because the answer depends on the pressure you started from, and nobody quoting the rule says so. Run the numbers from a 68°F starting point:

So “about 1 PSI per 10°F” is right for a car and badly low for a loaded truck. This calculator uses your own pressure rather than a fixed constant, which is why its answer drifts from the rule of thumb at the high end.

The same table runs the other way too. A road bike at 90 PSI loses close to 2 PSI for every 10°F, which is why a tyre pumped in a warm garage reads 7 low on a frosty start. The minimum here is set low enough to take bicycle pressures, so put 90 or 25 in and it works exactly the same. What pressure a bike tyre should be in the first place is the cycling tire pressure calculator, which works from rider weight and tyre width.

Cold reading versus hot reading

Driving heats the air inside the tire, and a tire checked after a highway run reads 4 to 6 PSI above its cold value. That is normal and expected. Do not bleed air out of a hot tire to reach the sticker number. You will be 4 to 6 PSI underinflated by morning, which is the single most common way people end up driving on soft tires all winter.

What being off actually costs you

• 10 PSI underinflated: roughly 2.5% worse fuel economy, wear concentrated on both shoulders, and a lot more heat in the sidewall. Heat is what destroys tires.
• 5 PSI overinflated: about 1% better fuel economy, a harsh ride, a smaller contact patch, and wear down the center of the tread.
• Max sidewall pressure: the structural ceiling of the tire, not a target. Running there gives you the worst of the overinflation list and no benefit.

Worked example

An SUV door sticker calls for 35 PSI. The pressure was set on a 40°F morning. Summer arrives and the same tire sits at 90°F, a change of +50°F.

Starting absolute: 35 + 14.7 = 49.7 PSI at 499.67 R. At 549.67 R: 49.7 × 549.67 / 499.67 = 54.67 PSI absolute. Back to gauge: 54.67 - 14.7 = 40.0 PSI.

That 5 PSI rise is the tire behaving correctly, not a fault. The thing to do in July is re-check cold and set it back to 35, not to let air out of a warm tire in a parking lot.

Check pressure monthly and before any long trip, with a digital gauge rather than the pencil type. The pencil gauges sold at gas stations are commonly 2 to 3 PSI out, which is most of the effect this page is about.