Every 4WD forum argues about the “correct” touring pressure, but the honest answer is that it depends on your vehicle, your load and your tyres. The four-pounds rule (also called the 4 psi rule) sidesteps the guesswork. Instead of chasing a magic number, it lets your tyres tell you whether the cold pressure you chose is actually right for how you’re loaded and how you drive. It’s one of the most useful habits an Australian tourer can learn, and it costs nothing but a gauge and an hour on the highway.

What is the four-pounds rule?

A tyre heats up as it rolls, because the sidewalls and tread flex thousands of times a minute. That flexing turns into heat, and heat raises the air pressure inside. The four-pounds rule uses that rise as a diagnostic. If a correctly inflated tyre is doing the right amount of flexing, it will gain about 4 psi between cold (parked) and hot (after a solid highway run). Too little air and it flexes more, runs hotter and gains more than 4 psi. Too much air and it barely flexes, stays cooler and gains less than 4 psi.

Because the rule reads the tyre’s own behaviour, it automatically accounts for your real-world load — a fully kitted touring rig with a loaded roof rack and a camper on the back needs more air than the same vehicle empty, and the 4 psi test finds that number for you. It pairs naturally with knowing the difference between your placard pressure and the sidewall maximum.

How do you actually do the 4 psi test?

Step by step

Run the test the way you’ll actually travel — loaded, on your normal touring tyres, in typical conditions.

1. Set your tyres cold. “Cold” means parked at least three hours or driven less than about 2 km. Choose a sensible starting pressure — often your on-road highway figure, e.g. around 38–40 psi for a loaded dual-cab (use it as a guide only).
2. Note the ambient temperature and the exact cold pressure of each tyre.
3. Drive at highway speed for at least 30–60 minutes so the tyres reach a stable working temperature.
4. Re-check each tyre straight away, while hot. Do not let them cool and do not bleed air out.
5. Compare the rise. Adjust your cold starting pressure up or down next time and repeat until the hot rise sits near 4 psi.

Always measure and adjust the target when the tyres are cold, not hot — you change the cold starting number between tests, never the hot reading.

What does the pressure rise tell you?

The numbers don’t have to be exact — anywhere close to 4 psi is fine. The point is the trend: a big rise is a warning that the tyre is working too hard.

Why does the rule work?

Radial tyres are designed to shed heat through contact with the road. At the correct pressure, the sidewalls flex just enough to grip and cushion without overheating. Drop the pressure too far and the sidewall flexes excessively, generating heat faster than the tyre can dump it. Left unchecked at speed, that heat can lead to delamination and a blowout — the classic cause of a highway tyre failure on an under-inflated, overloaded rig. This is exactly why under-inflation is the dangerous fault and why airing down for sand or dirt is only ever temporary.

When should you use the four-pounds rule?

The rule is a bitumen and formed-road tool: it sets your correct touring pressure for highway and hard-pack running. It is not how you set aggressive off-road pressures. For soft surfaces you deliberately air down well below the 4 psi figure to lengthen the contact patch — see our guides to sand-driving pressures and corrugated dirt roads. Once you’re back on the blacktop, re-inflate and, if you’ve changed your load, re-run the 4 psi test.

Also remember that ambient heat matters. On a 40°C day pressures already sit higher, and driving adds more — our explainer on summer heat and tyre pressure covers why you should never bleed hot pressure down to your cold target.

Common mistakes to avoid

• Bleeding air when tyres are hot. The rise is normal and expected — releasing it leaves you dangerously low once they cool.
• Testing empty then touring loaded. Always test in your real travel configuration.
• Chasing a single “internet” number. Your rig isn’t the one in the forum post.
• Skipping a decent gauge. A quality, accurate gauge (and ideally a good compressor) makes the whole method reliable. A TPMS can help you watch the hot pressures in real time.

Frequently asked questions

Is the four-pounds rule the same as the four-pound rule?

Yes. “Four pounds” refers to 4 psi (pounds per square inch). Some call it the 4 psi rule; it’s the same method.

How long do I need to drive for the test?

At least 30 minutes at highway speed, though up to an hour gives a more stable reading. The tyres need to reach normal working temperature.

Does the rule work for caravans and trailers?

Yes — it’s a good check for a loaded van or camper too. Test the trailer tyres the same way. See our caravan tyre pressure guide for starting figures.

What if my tyres gain way more than 4 psi even at high pressure?

Persistent large rises can point to overloading beyond the tyre’s rating, a load rating that’s too low, or a developing fault. Confirm your tyres meet the placard load rating and have them inspected.

Figures here are general guides for 2026. Always confirm against your vehicle’s tyre placard, your tyre manufacturer’s recommendations and, where relevant, your state road authority.

Authority sources: BFGoodrich Australia — off-road tyre pressure chart; Cooper Tires Australia — tyre pressure guide.