255/40R18 · passenger car · Bar
Recommended bar for 255/40R18
- Size
- 255/40R18
- Class
- passenger car
- Planning PSI
- 37
- Planning bar
- 2.55
bar
2.55
37 psi · 255 kPa · not a second factory sticker
255/40R18 · 37 psi · 2.55 bar · 255 kPa — estimates. Placard wins.
A 255/40R18 C-type or LT carcass can live far above a hatchback bar figure. This page’s 2.55 bar sketch is for the passenger car class, not every car that can physically mount the tyre. 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.
Bar, PSI and kPa
1 bar ≈ 14.5 psi; 1 psi ≈ 6.89 kPa. Planning midpoint for 255/40R18: 37 psi = 2.55 bar = 255 kPa.
| Bar | PSI | kPa |
|---|---|---|
| 2.28 | 33 | 228 |
| 2.34 | 34 | 234 |
| 2.41 | 35 | 241 |
| 2.48 | 36 | 248 |
| 2.55 | 37 | 255 |
| 2.62 | 38 | 262 |
| 2.69 | 39 | 269 |
| 2.76 | 40 | 276 |
| 2.83 | 41 | 283 |
| 2.90 | 42 | 290 |
| 2.96 | 43 | 296 |
| 3.03 | 44 | 303 |
| 3.10 | 45 | 310 |
| 3.17 | 46 | 317 |
| 3.24 | 47 | 324 |
| 3.31 | 48 | 331 |
| 3.38 | 49 | 338 |
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 | 37 psi / 2.55 bar / 255 kPa | 38 psi / 2.62 bar / 262 kPa |
| Motorway + luggage (sketch) | 38 psi / 2.62 bar / 262 kPa | 39 psi / 2.69 bar / 269 kPa |
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.
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.
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
- 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.
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 |
Keep 255/40R18 on the printed band for that car; 37 psi is a size sketch, not a second VIN label.
Questions about 255/40R18
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.
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.
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.
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.
Handbook: placard · bar vs PSI · cold inflation · all sizes