255/40R17 · passenger car · Overview
Recommended pressure for 255/40R17 tyres
- Size
- 255/40R17
- Class
- passenger car
- Planning PSI
- 37
- Planning bar
- 2.55
255/40R17
37 psi / 2.55 bar
255 kPa · estimates for this size class
255/40R17 · 37 psi · 2.55 bar · 255 kPa — estimates. Placard wins.
255/40R17 is a passenger car size (255 mm section, 40 % aspect, 17-inch rim). A planning cold figure around 37 psi / 2.55 bar / 255 kPa is a starting map, not a second factory sticker. Low-profile sport or plus-size wheel. These rows are estimates interpolated for the size, not OEM for a named VIN. 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 255/40R17 means
Read 255/40R17 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 sport or plus-size wheel. Rim diameter 17 in; section 255 mm. Estimates on this page assume a typical load index for the passenger car class.
Estimated 255/40R17 pressure by load
Load moves the rear first on a passenger car. If the factory placard already has a laden column, use that — not this sketch for 255/40R17.
| Condition | Front (est.) | Rear (est.) |
|---|---|---|
| Driver only / unladen | 37 psi / 2.55 bar / 255 kPa | 37 psi / 2.55 bar / 255 kPa |
| Typical occupancy | 38 psi / 2.62 bar / 262 kPa | 39 psi / 2.69 bar / 269 kPa |
| Motorway + luggage (sketch) | 39 psi / 2.69 bar / 269 kPa | 40 psi / 2.76 bar / 276 kPa |
PSI, bar and kPa
Use the table, then set a cold gauge. 255 kPa is 100 × 2.55 bar on this size.
| PSI | Bar | kPa |
|---|---|---|
| 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 |
| 49 | 3.38 | 338 |
Placard versus sidewall
If a forum says “all 255/40R17 run 37 psi”, ignore it. A Qashqai and a Golf can share a size and not share a sticker.
Cars often seen on 255/40R17
255/40R17 is a valid metric size in this register even if it is not on the curated OEM shortlist. Treat the 37 psi / 2.55 bar figure as a class estimate and confirm on the vehicle that actually wears it.
How to set 255/40R17
- Measure before the school run, not after the motorway.
- Write LF / RF / RR / RL. Do not keep 255/40R17 numbers in your head.
- Add air in short bursts toward the placard, using 37 psi only as a map.
- If one 255/40R17 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 37 psi as a 20 °C baseline for 255/40R17.
| Ambient | Gauge | Bar | kPa | Note |
|---|---|---|---|---|
| −5 °C (hard frost) | 34 psi | 2.34 bar | 234 kPa | Gauge reads low; do not overfill once warm |
| 0 °C | 35 psi | 2.41 bar | 241 kPa | Gauge reads low; do not overfill once warm |
| 8 °C (UK winter morning) | 36 psi | 2.48 bar | 248 kPa | Gauge reads low; do not overfill once warm |
| 20 °C (set cold here) | 37 psi | 2.55 bar | 255 kPa | Baseline for this size sketch |
| 30 °C | 39 psi | 2.69 bar | 269 kPa | Do not bleed a hot 255/40R17 |
| 38 °C (heatwave tarmac) | 40 psi | 2.76 bar | 276 kPa | Do not bleed a hot 255/40R17 |
A cheap gauge and a monthly habit will do more for a set of 255/40R17 than nitrogen upsells.
Questions about 255/40R17
Is 255/40R17 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 255/40R17 in kPa?
255 kPa is the planning midpoint (100 kPa = 1 bar). 262 kPa is one PSI higher.
When does TPMS warn on 255/40R17?
Direct TPMS typically flags about 25% below target — near 28 psi if you were aiming at 37. Fill to the sticker, not to the lamp.
Can I run 255/40R17 at a lower pressure for comfort?
Not as a habit. A soft 255/40R17 cooks the shoulders and dulls steering. Keep load index and stay on the placard.
Is the sidewall number correct for 255/40R17?
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