PSI to kPa for Truck Tyres, Brakes and Load Tables

Fleet Air Pressures When the Spec Sheet Is Metric

A shop gauge on a Class 8 tractor reads in psi. The tyre manufacturer's load-inflation table, the trailer built for a European operator and the imported air-dryer manual all quote kilopascals. Somewhere between the placard, the treadle gauge and the paperwork, a technician has to make those two columns agree — for steer, drive and trailer tyres, and for every threshold in the air-brake charging system.

Conversion factor: 1 psi = 6.894757293 kPa. A steer tyre set to 110 psi cold is 758.42 kPa, which is the figure you look up in a metric load-inflation table to confirm the axle rating.

Where the Numbers Show Up on a Truck

Steer axle inflation

Steer tyres carry the heaviest load per tyre on the vehicle — one each side rather than a pair — so they sit at the top of the inflation range, often 110 to 120 psi.

Duals on drives and trailer

Two tyres share each wheel end, so each needs less pressure for the same axle weight — and both in a pair must be set to the same figure to share that weight evenly.

The charging range

The governor decides when the compressor loads and unloads. Cut-out sits near the top of the dash gauge, cut-in some 20 to 25 psi lower, and the system cycles between the two all day.

Warning and spring-brake points

A falling reservoir crosses the low-air warning first, then the point where the spring brakes set themselves. Those thresholds appear in regulations as psi and in imported manuals as kPa.

Working a Pre-Trip Reading Through the Converter

Nothing here needs a calculator app or a printed chart taped inside the toolbox lid — type the reading and read the other column.

Put the gauge reading in the psi box

110, 100, 130 — whatever the tyre gauge or the dash showed. Kilopascals appear beside it while you are still typing, and a comma instead of a dot is accepted if that is how your keypad works.

Line it up with the metric table

Load-inflation tables published in kPa usually step in 20 kPa increments, so round to the nearest listed row rather than inventing a value between two entries.

Lift the bare figure for the inspection record

Each field carries its own copy button that hands over the number alone — no unit, no spacing — which is what a maintenance system field or a spreadsheet column will accept. Pressing Ctrl + C while the cursor sits in a field does the same job.

Go the other way for a metric-built unit

Tap the swap arrows (↔) and the page runs kPa → psi instead, for trailers and dryers delivered with metric plates. Done by hand it is a multiply by 0.145037738: 830 kPa lands just past 120 psi.

Everything on a truck is gauge pressure. Tyre gauges, dash gauges and governor settings all read above atmospheric, so a converted kPa figure is still a gauge figure — do not line it up against an absolute value taken from a physics table.

Commercial Tyre and Air-Brake Pressure Chart

Typical settings and switching points on a tractor-trailer, in the psi a technician sees on the gauge and the kPa printed in metric documentation. Always defer to the vehicle placard, the tyre maker's table and the brake manufacturer's specification for the unit in front of you.

Setting or threshold	Where it applies	psi	kPa
Steer tyre, heavy front axle	315/80R22.5 near its load limit	120 psi	827.37 kPa
Steer tyre, common fleet setting	Cold, normal front-axle weight	110 psi	758.42 kPa
Drive tyre, dual	11R22.5 or 295/75R22.5 duals	100 psi	689.48 kPa
Trailer tyre, light load	Duals under a lightly loaded van	90 psi	620.53 kPa
Governor cut-out	Compressor unloads, top of the range	130 psi	896.32 kPa
Governor cut-in	Compressor starts pumping again	105 psi	723.95 kPa
Full service application	Treadle to the floor, delivered to the chambers	90 psi	620.53 kPa
Low-air warning	Light and buzzer come on	60 psi	413.69 kPa
Spring brakes apply themselves	Parking brakes set as pressure keeps falling	20–45 psi	137.90–310.26 kPa

Two of those are worth memorising in both units because inspectors quote them: the warning at 60 psi (413.69 kPa) and the ceiling a governor is not allowed to exceed, 135 psi (930.79 kPa). Leak-rate limits scale the same way — a 3 psi drop in one minute is 20.68 kPa, and 4 psi is 27.58 kPa.

What the Page Gives a Technician

Either box accepts the shop reading

Both fields are editable and each drives the other, so a full set of wheel positions can be walked through without clearing anything between tyres.

Flip the direction for imported equipment

One tap on the swap arrows turns the page into a kPa → psi tool for a metric-plated trailer, air dryer or suspension spec.

Bar and kg/cm² sit in the same dropdowns

Both unit lists are searchable and cover the whole pressure family, so an air-suspension figure in bar or an old gauge marked kg/cm² is handled on the same screen.

A figure fit for the work order

Results carry up to eight decimals with thousands spaced out for readability, and the copy button strips all of that formatting back to a plain number.

Questions From the Shop Floor

Why do steer tyres run higher than the drives and trailer tyres?

Load per tyre. A steer axle puts its whole weight on two tyres, while a drive or trailer axle spreads a similar weight over four. Inflation follows the weight each individual tyre carries, which is why a steer commonly sits at 110 psi (758.42 kPa) or above while duals are content nearer 100 psi (689.48 kPa). Weigh the axle, then read the tyre maker's table for that size and load.

What should governor cut-in and cut-out read on a healthy system?

Cut-out typically lands between about 120 and 135 psi — 827.37 to 930.79 kPa — with cut-in roughly 20 to 25 psi below it, so a system unloading at 130 psi (896.32 kPa) should start pumping again near 105 to 110 psi (723.95 to 758.42 kPa). A build from cut-in to cut-out that takes far longer than the manufacturer allows points at the compressor, the governor or a leak rather than at the gauge.

The low-air buzzer came on mid-route — at what pressure does it trip?

The warning has to activate before reservoir pressure falls below 60 psi, which is 413.69 kPa; where cut-out is high, the trip point may instead be set at half of cut-out, which is higher still. Treat it as a signal to stop while you still have service brakes. Keep losing air and the spring brakes will set themselves somewhere between roughly 20 and 45 psi (137.90 to 310.26 kPa), wherever the vehicle happens to be at that moment.

My load-inflation table only lists kPa — how do I use it with a psi gauge?

Find the row for your tyre size and the axle weight you actually scaled, then bring that kPa figure back to psi here and set the gauge to it. Metric tables step in 20 kPa jumps, a shade under 3 psi, so you will normally round up to the next listed row instead of sitting between two of them. Never drop below the placard minimum just because the table permits it for a lighter axle than you are running.

How closely do the two tyres in a dual pair need to match?

Within about 5 psi — roughly 34.47 kPa — is the figure fleets normally work to, and tighter is better. A mismatch makes the higher tyre stand taller, so it takes more than its share of the load and drags the softer one along; on a metric table that same gap is a real difference in rated capacity. Check both tyres of a pair at once with the same gauge, and use a dual-foot chuck or valve extensions so the inner tyre never gets skipped.

PSI to Kilopascals