235/40R18 · passenger car · kPa
235/40R18 kPa inflation table
- Size
- 235/40R18
- Class
- passenger car
- Planning PSI
- 36
- Planning bar
- 2.48
kPa
248
2.48 bar · 36 psi · 100 kPa = 1 bar
235/40R18 · 36 psi · 2.48 bar · 248 kPa — estimates. Placard wins.
Kilopascals show up on some Asian stickers and ISO sheets. 248 kPa is the planning midpoint for 235/40R18 (2.48 bar / 36 psi). 100 kPa = 1 bar. 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.
kPa, bar and PSI
Convert around the 235/40R18 band. 2.48 bar is about 36 psi — not 23, not 33 unless the maths says so.
| kPa | Bar | PSI |
|---|---|---|
| 221 | 2.21 | 32 |
| 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 |
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 | 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 |
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.
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.
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 |
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.
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.
A cheap gauge and a monthly habit will do more for a set of 235/40R18 than nitrogen upsells.
Questions about 235/40R18
Can I run 235/40R18 at a lower pressure for comfort?
Not as a habit. A soft 235/40R18 cooks the shoulders and dulls steering. Keep load index and stay on the placard.
What PSI should 235/40R18 tyres be?
A passenger car planning figure is about 36 psi cold (2.48 bar / 248 kPa). Confirm on the vehicle placard — two cars can share 235/40R18 and not share a sticker.
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.
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.
What is 235/40R18 in kPa?
248 kPa is the planning midpoint (100 kPa = 1 bar). 255 kPa is one PSI higher.
Handbook: placard · bar vs PSI · cold inflation · all sizes