225/40R18 · passenger car · kPa
225/40R18 tyre pressure in kPa
- Size
- 225/40R18
- Class
- passenger car
- Planning PSI
- 36
- Planning bar
- 2.48
kPa
248
2.48 bar · 36 psi · 100 kPa = 1 bar
225/40R18 · 36 psi · 2.48 bar · 248 kPa — estimates. Placard wins.
If the gauge only speaks kPa, 248 kPa is the sketch for this passenger car size. Fine-tune to the vehicle that actually wears 225/40R18. A 10 °C overnight drop takes roughly 1 psi (0.07 bar) off a closed 225/40R18. 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 225/40R18 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 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 |
What 225/40R18 means
225/40R18 means a 225 millimetre section width, a sidewall that is 40 percent of that width, on a 18-inch rim. The R is radial construction.
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
This website maps size searches such as “225/40R18 psi”. It does not replace the VIN label. If the two disagree, the label wins.
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 |
How to set 225/40R18
- Park so the 225/40R18 tyres are cold (three hours, or less than about a mile).
- Read the door-pillar placard for that car — not the 225/40R18 sidewall maximum.
- If the sticker is missing, 36 psi / 2.48 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.
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.
A cheap gauge and a monthly habit will do more for a set of 225/40R18 than nitrogen upsells.
Questions about 225/40R18
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 PSI should 225/40R18 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 225/40R18 and not share a sticker.
Can I run 225/40R18 at a lower pressure for comfort?
Not as a habit. A soft 225/40R18 cooks the shoulders and dulls steering. Keep load index and stay on the placard.
When does TPMS warn on 225/40R18?
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.
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.
Handbook: placard · bar vs PSI · cold inflation · all sizes