Your correct PSI, the temperature adjustment, and what wrong pressure costs
| Running at… | Fuel penalty | Tire life | Safety |
|---|
Tire pressure is the highest-return 90 seconds in car care, and most drivers get the basics wrong: the correct number is on the door jamb placard (not the tire's sidewall — that's the tire's maximum, not your car's spec), it must be measured cold, and it silently drops 1 PSI per 10°F of temperature — which is why every first cold snap fills tire shops with TPMS-light appointments. This calculator gives your set-to number, temperature-adjusted, and prices what riding low actually costs.
| Rule | Why |
|---|---|
| Door placard, not sidewall | The placard is your car's engineered spec (ride, handling, load); the sidewall's 'MAX 51 PSI' is the tire's structural ceiling |
| Measure cold | Driving heats tires +4–6 PSI; a 'perfect' warm reading is actually low |
| 1 PSI per 10°F | Physics — set 35 at 70°F and a 30°F morning reads 31: officially underinflated without a single leak |
| Check monthly + before trips | Tires naturally lose ~1 PSI/month through the rubber |
| TPMS ≠ maintenance | The light triggers at 25% low — you've been burning fuel and tread for weeks by then |
The sidewall states the tire's maximum safe pressure; the door placard states what YOUR car was engineered for — ride, handling, braking and load balance. Inflating to sidewall max over-hardens the tire and wears the centers. Placard wins, always.
Probably just physics: a 30-40°F drop takes 3-4 PSI, crossing the 25%-low threshold on tires that were merely 'a bit low.' Top up to the calculator's temperature-adjusted target; if one tire is notably lower than the rest, that one has a real leak.
Never — warm readings are SUPPOSED to be 4-6 PSI over your cold setting. Bleeding warm tires to the placard leaves them underinflated when cold. All targets are cold targets.
Marginally more stable pressure, mostly irrelevant at street level — race cars and aircraft care; your commuter doesn't. Free air checked monthly beats $10/tire nitrogen checked never.
Compact spares typically want 60 PSI (printed on them) and are the most-neglected tire in every car — check it twice a year; a flat spare is two problems.
They run whatever THEIR placard says — often 38-45 PSI to carry battery weight and cut rolling resistance. Same rules: placard, cold, temperature-adjusted. EV tire wear makes the discipline pay even faster.
Yes — everything computes locally in your browser.
Placard number, cold reading, temperature math, monthly habit — four rules that buy better braking, longer tires and free MPG. The gauge costs ten dollars; the ignorance costs more every month.