255/40R18 · passenger car · kPa
255/40R18 kPa inflation table
- Size
- 255/40R18
- Class
- passenger car
- Planning PSI
- 37
- Planning bar
- 2.55
kPa
255
2.55 bar · 37 psi · 100 kPa = 1 bar
255/40R18 · 37 psi · 2.55 bar · 255 kPa — estimates. Placard wins.
If the gauge only speaks kPa, 255 kPa is the sketch for this passenger car size. Fine-tune to the vehicle that actually wears 255/40R18. Under-inflation cooks the shoulders of a 255/40R18; over-inflation wears the centre rib and bangs every cat’s eye. 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.
kPa, bar and PSI
Convert around the 255/40R18 band. 2.55 bar is about 37 psi — not 23, not 33 unless the maths says so.
| kPa | Bar | PSI |
|---|---|---|
| 228 | 2.28 | 33 |
| 234 | 2.34 | 34 |
| 241 | 2.41 | 35 |
| 248 | 2.48 | 36 |
| 255 | 2.55 | 37 |
| 262 | 2.62 | 38 |
| 269 | 2.69 | 39 |
| 276 | 2.76 | 40 |
| 283 | 2.83 | 41 |
| 290 | 2.90 | 42 |
| 296 | 2.96 | 43 |
| 303 | 3.03 | 44 |
| 310 | 3.10 | 45 |
| 317 | 3.17 | 46 |
| 324 | 3.24 | 47 |
| 331 | 3.31 | 48 |
| 338 | 3.38 | 49 |
Estimated 255/40R18 pressure by load
Plus-sizing to 255/40R18 from a taller OEM tyre usually keeps the same placard if the load index is equal or higher.
| 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 |
What 255/40R18 means
255/40R18 means a 255 millimetre section width, a sidewall that is 40 percent of that width, on a 18-inch rim. The R is radial construction.
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.
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 |
How to set 255/40R18
- Park so the 255/40R18 tyres are cold (three hours, or less than about a mile).
- Read the door-pillar placard for that car — not the 255/40R18 sidewall maximum.
- If the sticker is missing, 37 psi / 2.55 bar is only a passenger car planning figure for this size.
- Set all four with a calibrated gauge. A 3 psi miss on one corner shows up as a wander.
- Refit valve caps and reset TPMS if the handbook asks for a relearn.
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.
If the door sticker and this 255/40R18 page disagree, the sticker wins.
Questions about 255/40R18
Is 255/40R18 the same as a plus-size swap?
Only if that is the size you fitted. Overall diameter should stay close to OEM. Pressure still follows the car, with load index at or above original.
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 PSI should 255/40R18 tyres be?
A passenger car planning figure is about 37 psi cold (2.55 bar / 255 kPa). Confirm on the vehicle placard — two cars can share 255/40R18 and not share a sticker.
Should I bleed pressure after a motorway run?
No. Heat raises the gauge. Set cold only. Bleeding a hot tyre leaves it low the next morning.
Handbook: placard · bar vs PSI · cold inflation · all sizes