225/40R18 · passenger car · PSI
What PSI for 225/40R18 tyres?
- Size
- 225/40R18
- Class
- passenger car
- Planning PSI
- 36
- Planning bar
- 2.48
psi
36
2.48 bar · 248 kPa · passenger car planning figure
225/40R18 · 36 psi · 2.48 bar · 248 kPa — estimates. Placard wins.
If someone typed “225/40R18 psi”, they want Imperial. Start near 36 psi on an unladen passenger car, then confirm the vehicle placard. Sidewall max is not the target. Under-inflation cooks the shoulders of a 225/40R18; 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.
Estimated 225/40R18 pressure by load
These 225/40R18 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 |
PSI, bar and kPa
1 bar ≈ 14.5 psi; 1 psi ≈ 6.89 kPa. Planning midpoint for 225/40R18: 36 psi = 2.48 bar = 248 kPa.
| PSI | Bar | kPa |
|---|---|---|
| 32 | 2.21 | 221 |
| 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 |
What 225/40R18 means
Read 225/40R18 as width / aspect R rim. Nothing in the size string is a pressure. Pressure lives on the car, not moulded as 205 or 55.
Low-profile 18-inch. Rim diameter 18 in; section 225 mm. Estimates on this page assume a typical load index for the passenger car class.
Placard versus sidewall
If a forum says “all 225/40R18 run 36 psi”, ignore it. A Qashqai and a Golf can share a size and not share a sticker.
Cars often seen on 225/40R18
These catalogue models are often seen on 225/40R18 or a close OEM neighbour. Open the vehicle chart for axle-specific PSI and bar.
How to set 225/40R18
- Measure before the school run, not after the motorway.
- Write LF / RF / RR / RL. Do not keep 225/40R18 numbers in your head.
- Add air in short bursts toward the placard, using 36 psi only as a map.
- If one 225/40R18 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 225/40R18.
| 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 225/40R18 |
| 38 °C (heatwave tarmac) | 39 psi | 2.69 bar | 269 kPa | Do not bleed a hot 225/40R18 |
Keep 225/40R18 on the printed band for that car; 36 psi is a size sketch, not a second VIN label.
Questions about 225/40R18
Is the sidewall number correct for 225/40R18?
No. Sidewall PSI or kPa is a maximum for the carcass. Operating pressure is the door-pillar placard.
Is 225/40R18 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 225/40R18 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.
What is 225/40R18 in kPa?
248 kPa is the planning midpoint (100 kPa = 1 bar). 255 kPa is one PSI higher.
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