Of the two ways to get tyre pressure wrong, underinflation is the dangerous one. Overinflation mostly costs you comfort and even wear; running too low builds heat inside the tyre, and heat is what destroys them. With Australian summer bitumen already pushing tyre temperatures high, a soft tyre has a much smaller safety margin. Here’s what actually happens when your pressures drop below the placard figure.

What does underinflation do to a tyre?

A tyre with too little air can’t hold its shape under load, so the sidewalls flex more with every rotation. That constant flexing generates heat, and it pushes the load onto the outer edges of the tread rather than the full width.

Why is underinflation a blowout risk?

This is the one that matters most. When a tyre is low, its sidewalls flex far beyond their design, and that repeated flexing turns into heat. On a long, hot Australian highway run, the temperature keeps climbing until the rubber and its internal belts can separate — a sudden, violent blowout, usually at the worst possible speed. It’s why airing down for sand is strictly a low-speed, temporary measure that you must reverse before sealed roads. If it ever happens to you, know what to do if you blow a tyre at highway speed.

Does low pressure really wear tyres faster?

Yes, and in a distinctive pattern. With the load pushed onto the shoulders, both outer edges of the tread wear ahead of the centre — the mirror image of overinflation’s centre wear. If you’re seeing it on the inside edge or the outside edge only, that’s more likely an alignment issue; even wear on both shoulders across all four tyres points to chronic underinflation. Either way you replace tyres sooner, which pushes up your cost per kilometre.

How much fuel does underinflation waste?

A soft tyre deforms more as it rolls, and that lost energy shows up at the bowser. As a guide, fuel economy drops by around 0.3% for every psi below the recommended pressure, so a set that’s 6–8 psi low can cost you a couple of percent on fuel — every tank, for as long as you leave it. That’s the cheapest efficiency fix there is. More detail in tyre pressure and fuel economy.

Why does the car feel worse to drive?

Underinflated tyres make the steering feel heavy and vague, the car floats and leans more through corners, and braking distances stretch out because the tread isn’t sitting flat on the road. It’s easy to write off as “the car getting old” when it’s really just low tyres. A quick pressure check often restores the sharpness for nothing.

What causes tyres to run low?

Natural permeation (1–2 psi a month), a drop in temperature (about 1 psi per 5–6°C, which is why the warning light appears on cold mornings), a slow puncture, a valve leak, or simply carrying a heavy load on standard pressures. The fix is the same: check monthly, cold, and reset to placard — adding air for loads or towing where the placard’s laden column calls for it.

Frequently asked questions

How low is too low? Anything below your placard pressure is too low. A tyre more than about 25% under is genuinely unsafe and is roughly where a TPMS finally warns you.

Can I drive on a slightly low tyre? A short, slow trip to add air is fine. Don’t drive far or fast on a noticeably soft tyre — heat is the danger.

Is underinflation worse than overinflation? For safety, yes. Underinflation’s heat build-up and blowout risk make it the more dangerous fault.

Will low pressure damage the wheel? It can. A very low tyre lets the rim take impacts directly and can be pinched against the road, damaging both tyre and wheel.

Bottom line

Underinflation is the dangerous pressure fault: it builds heat, wears your edges, wastes fuel and can end in a blowout. Keep every tyre at its placard pressure, check cold and monthly, and never leave a soft tyre “for later.”

Sources: RACV — tyre pressure guide. Figures are a guide; confirm your vehicle’s pressures on the placard.

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