Tire Pressure Changes With Load — and Almost Nobody Adjusts It

The correct pressure for an empty truck is not the correct pressure for a loaded one. Getting this wrong is the most common cause of work-truck tire failure.

On a passenger car, pressure is one number and you set it and forget it. On a work truck that runs empty half the week and loaded the other half, one number is wrong at least half the time.

Why load changes the requirement

A tire supports load through the air inside it, not through the rubber. More load needs more pressure to maintain the same sidewall deflection. Run a loaded tire at empty-truck pressure and the sidewall flexes further than designed — which is the exact condition that generates heat and separates belts.

The reverse is also a problem. An empty truck at loaded pressure rides badly, wears the centre of the tread, and has less contact patch and therefore less grip.

Where the numbers come from

• The door jamb placard gives the manufacturer's recommendation, generally for a normally loaded vehicle.
• Many trucks list two figures — one for light load, one at maximum. Use the one that matches what you are actually carrying.
• The tire sidewall number is a maximum, not a recommendation. Setting to it because it is printed there is a common mistake.
• Load and inflation tables from the tire manufacturer give the correct pressure for a measured axle weight, which is the precise answer for a fleet.

Temperature is the other variable

Pressure changes roughly one psi for every ten degrees Fahrenheit of ambient change. A West Texas cold front dropping thirty degrees overnight takes about three pounds out of every tire. That is why the TPMS lights across a whole parking lot come on the same morning.

A practical routine for a work truck

1. 1Own a decent gauge and keep it in the truck. Pencil gauges are not accurate enough to be useful.
2. 2Check cold, monthly, and whenever the load changes significantly.
3. 3Check after any large temperature swing.
4. 4Include the spare and the trailer.
5. 5Set to the placard figure appropriate for the load, not to the sidewall maximum.

For fleets we set this up when the units come through the shop — pressures checked and corrected on every truck, which takes far less time than one blowout.