255/40R18 · passenger car · Overview
255/40R18: PSI, bar and kPa
- Size
- 255/40R18
- Class
- passenger car
- Planning PSI
- 37
- Planning bar
- 2.55
255/40R18
37 psi / 2.55 bar
255 kPa · estimates for this size class
255/40R18 · 37 psi · 2.55 bar · 255 kPa — estimates. Placard wins.
255/40R18 is a passenger car size (255 mm section, 40 % aspect, 18-inch rim). A planning cold figure around 37 psi / 2.55 bar / 255 kPa is a starting map, not a second factory sticker. Wide 18-inch / EV aero. These rows are estimates interpolated for the size, not OEM for a named VIN. 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.
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.
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 |
PSI, bar and kPa
1 bar ≈ 14.5 psi; 1 psi ≈ 6.89 kPa. Planning midpoint for 255/40R18: 37 psi = 2.55 bar = 255 kPa.
| 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 |
Placard versus sidewall
If a forum says “all 255/40R18 run 37 psi”, ignore it. A Qashqai and a Golf can share a size and not share a sticker.
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 |
A cheap gauge and a monthly habit will do more for a set of 255/40R18 than nitrogen upsells.
Questions about 255/40R18
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.
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.
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 kPa?
255 kPa is the planning midpoint (100 kPa = 1 bar). 262 kPa is one PSI higher.
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.
Handbook: placard · bar vs PSI · cold inflation · all sizes