255/40R18 · passenger car · PSI
255/40R18 PSI chart (37 psi planning figure)
- Size
- 255/40R18
- Class
- passenger car
- Planning PSI
- 37
- Planning bar
- 2.55
psi
37
2.55 bar · 255 kPa · passenger car planning figure
255/40R18 · 37 psi · 2.55 bar · 255 kPa — estimates. Placard wins.
If someone typed “255/40R18 psi”, they want Imperial. Start near 37 psi on an unladen passenger car, then confirm the vehicle placard. Sidewall max is not the target. A 10 °C overnight drop takes roughly 1 psi (0.07 bar) off a closed 255/40R18. Low-profile {sz} does not want a “softer” pressure to recover ride. Stay at or slightly above the car’s original placard if load index is kept.
Estimated 255/40R18 pressure by load
Load moves the rear first on a passenger car. If the factory placard already has a laden column, use that — not this sketch for 255/40R18.
| Condition | Front (est.) | Rear (est.) |
|---|---|---|
| Driver only / unladen | 37 psi / 2.55 bar / 255 kPa | 37 psi / 2.55 bar / 255 kPa |
| Typical occupancy | 38 psi / 2.62 bar / 262 kPa | 39 psi / 2.69 bar / 269 kPa |
| Motorway + luggage (sketch) | 39 psi / 2.69 bar / 269 kPa | 40 psi / 2.76 bar / 276 kPa |
PSI, bar and kPa
Use the table, then set a cold gauge. 255 kPa is 100 × 2.55 bar on this size.
| PSI | Bar | kPa |
|---|---|---|
| 33 | 2.28 | 228 |
| 34 | 2.34 | 234 |
| 35 | 2.41 | 241 |
| 36 | 2.48 | 248 |
| 37 | 2.55 | 255 |
| 38 | 2.62 | 262 |
| 39 | 2.69 | 269 |
| 40 | 2.76 | 276 |
| 41 | 2.83 | 283 |
| 42 | 2.90 | 290 |
| 43 | 2.96 | 296 |
| 44 | 3.03 | 303 |
| 45 | 3.10 | 310 |
| 46 | 3.17 | 317 |
| 47 | 3.24 | 324 |
| 48 | 3.31 | 331 |
| 49 | 3.38 | 338 |
What 255/40R18 means
Read 255/40R18 as width / aspect R rim. Nothing in the size string is a pressure. Pressure lives on the car, not moulded as 205 or 55.
Wide 18-inch / EV aero. Rim diameter 18 in; section 255 mm. Estimates on this page assume a typical load index for the passenger car class.
Placard versus sidewall
The sidewall of a 255/40R18 may say 51 psi or 350 kPa. That is a carcass maximum. Operating pressure is the tyre-information placard on the driver’s door pillar.
Cars often seen on 255/40R18
These catalogue models are often seen on 255/40R18 or a close OEM neighbour. Open the vehicle chart for axle-specific PSI and bar.
How to set 255/40R18
- Measure before the school run, not after the motorway.
- Write LF / RF / RR / RL. Do not keep 255/40R18 numbers in your head.
- Add air in short bursts toward the placard, using 37 psi only as a map.
- If one 255/40R18 is 4 psi down, hunt for a nail before you blame the weather.
- Log date, ambient temperature and the four readings.
Temperature drift
Roughly 1 psi per 5–6 °C. The table uses 37 psi as a 20 °C baseline for 255/40R18.
| Ambient | Gauge | Bar | kPa | Note |
|---|---|---|---|---|
| −5 °C (hard frost) | 34 psi | 2.34 bar | 234 kPa | Gauge reads low; do not overfill once warm |
| 0 °C | 35 psi | 2.41 bar | 241 kPa | Gauge reads low; do not overfill once warm |
| 8 °C (UK winter morning) | 36 psi | 2.48 bar | 248 kPa | Gauge reads low; do not overfill once warm |
| 20 °C (set cold here) | 37 psi | 2.55 bar | 255 kPa | Baseline for this size sketch |
| 30 °C | 39 psi | 2.69 bar | 269 kPa | Do not bleed a hot 255/40R18 |
| 38 °C (heatwave tarmac) | 40 psi | 2.76 bar | 276 kPa | Do not bleed a hot 255/40R18 |
If the door sticker and this 255/40R18 page disagree, the sticker wins.
Questions about 255/40R18
Is the sidewall number correct for 255/40R18?
No. Sidewall PSI or kPa is a maximum for the carcass. Operating pressure is the door-pillar placard.
Can I run 255/40R18 at a lower pressure for comfort?
Not as a habit. A soft 255/40R18 cooks the shoulders and dulls steering. Keep load index and stay on the placard.
What is 255/40R18 in bar?
37 psi is 2.55 bar. Add a tenth of a bar if the placard’s laden column is higher, which is common on vans that wear this size.
What is 255/40R18 in kPa?
255 kPa is the planning midpoint (100 kPa = 1 bar). 262 kPa is one PSI higher.
When does TPMS warn on 255/40R18?
Direct TPMS typically flags about 25% below target — near 28 psi if you were aiming at 37. Fill to the sticker, not to the lamp.
Handbook: placard · bar vs PSI · cold inflation · all sizes