225/40R18 · passenger car · Overview
225/40R18: PSI, bar and kPa
- Size
- 225/40R18
- Class
- passenger car
- Planning PSI
- 36
- Planning bar
- 2.48
225/40R18
36 psi / 2.48 bar
248 kPa · estimates for this size class
225/40R18 · 36 psi · 2.48 bar · 248 kPa — estimates. Placard wins.
Searches for “225/40R18 pressure” usually want a number. Use 36 psi (2.48 bar) only as a sketch; the door-pillar placard on the car that wears this size still wins. 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.
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.
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 | 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 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 |
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
- 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.
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
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.
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.
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.
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.
Handbook: placard · bar vs PSI · cold inflation · all sizes