235/40R18 · passenger car · Bar
Recommended bar for 235/40R18
- Size
- 235/40R18
- Class
- passenger car
- Planning PSI
- 36
- Planning bar
- 2.48
bar
2.48
36 psi · 248 kPa · not a second factory sticker
235/40R18 · 36 psi · 2.48 bar · 248 kPa — estimates. Placard wins.
European stickers print bar. 235/40R18 often sits near 2.48 bar on a passenger car (36 psi / 248 kPa). 2.48 bar is not “36 divided by ten”. TPMS typically warns near 27 psi (about 25% below a 36 psi target). That is already a soft tyre. 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
Convert around the 235/40R18 band. 2.48 bar is about 36 psi — not 23, not 33 unless the maths says so.
| Bar | PSI | kPa |
|---|---|---|
| 2.21 | 32 | 221 |
| 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 |
Estimated 235/40R18 pressure by load
These 235/40R18 load steps are ± a few PSI from 36. They are not a licence to ignore a printed second column on a van or estate.
| Condition | Front (est.) | Rear (est.) |
|---|---|---|
| Driver only / unladen | 36 psi / 2.48 bar / 248 kPa | 36 psi / 2.48 bar / 248 kPa |
| Typical occupancy | 36 psi / 2.48 bar / 248 kPa | 37 psi / 2.55 bar / 255 kPa |
| Motorway + luggage (sketch) | 37 psi / 2.55 bar / 255 kPa | 38 psi / 2.62 bar / 262 kPa |
Placard versus sidewall
If a forum says “all 235/40R18 run 36 psi”, ignore it. A Qashqai and a Golf can share a size and not share a sticker.
What 235/40R18 means
235/40R18 means a 235 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 sport. Rim diameter 18 in; section 235 mm. Estimates on this page assume a typical load index for the passenger car class.
Cars often seen on 235/40R18
These catalogue models are often seen on 235/40R18 or a close OEM neighbour. Open the vehicle chart for axle-specific PSI and bar.
How to set 235/40R18
- Measure before the school run, not after the motorway.
- Write LF / RF / RR / RL. Do not keep 235/40R18 numbers in your head.
- Add air in short bursts toward the placard, using 36 psi only as a map.
- If one 235/40R18 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 36 psi as a 20 °C baseline for 235/40R18.
| Ambient | Gauge | Bar | kPa | Note |
|---|---|---|---|---|
| −5 °C (hard frost) | 33 psi | 2.28 bar | 228 kPa | Gauge reads low; do not overfill once warm |
| 0 °C | 34 psi | 2.34 bar | 234 kPa | Gauge reads low; do not overfill once warm |
| 8 °C (UK winter morning) | 35 psi | 2.41 bar | 241 kPa | Gauge reads low; do not overfill once warm |
| 20 °C (set cold here) | 36 psi | 2.48 bar | 248 kPa | Baseline for this size sketch |
| 30 °C | 38 psi | 2.62 bar | 262 kPa | Do not bleed a hot 235/40R18 |
| 38 °C (heatwave tarmac) | 39 psi | 2.69 bar | 269 kPa | Do not bleed a hot 235/40R18 |
If the door sticker and this 235/40R18 page disagree, the sticker wins.
Questions about 235/40R18
When does TPMS warn on 235/40R18?
Direct TPMS typically flags about 25% below target — near 27 psi if you were aiming at 36. Fill to the sticker, not to the lamp.
What is 235/40R18 in bar?
36 psi is 2.48 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 the sidewall number correct for 235/40R18?
No. Sidewall PSI or kPa is a maximum for the carcass. Operating pressure is the door-pillar placard.
What is 235/40R18 in kPa?
248 kPa is the planning midpoint (100 kPa = 1 bar). 255 kPa is one PSI higher.
Is 235/40R18 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