THE IDEAL GAS LAW

Your tyre contains air — a gas that follows basic physics. When temperature rises, air molecules move faster and push harder against the tyre walls. When it drops, they slow down and pressure decreases.

This isn't a rough estimate — it's physics. A tyre set to 35 PSI at 15°C will read approximately 38 PSI at 30°C and only 32 PSI at 0°C. The air inside hasn't leaked; it's simply expanded or contracted.

REAL-WORLD NZ EXAMPLES

Scenario	Set Temp	Current Temp	PSI Change
☀️ Summer morning → afternoon	18°C	35°C	+3.1 PSI
❄️ Autumn → winter morning	15°C	2°C	−2.4 PSI
🛞 Garage (cool) → hot road	12°C	40°C	+5.1 PSI
✓ Correct (check when cold)	15°C	15°C	0 PSI

THE NZ PROBLEM

An AA New Zealand survey of 150 vehicles at WOF inspections found that 48% had underinflated tyres. That's nearly half of all cars on our roads running pressures that increase fuel consumption, accelerate wear, reduce grip, and raise blowout risk.

Source: AA New Zealand Tyre Pressure Survey, aa.co.nz

NZ's variable climate makes this worse. A tyre set correctly in April can be 3-4 PSI low by June as temperatures drop. Most drivers don't adjust — they just wonder why their car feels sluggish and fuel economy has dropped.

HOW BLOWOUTS HAPPEN

When a tyre is underinflated, the sidewall flexes more than designed. Each flex generates heat. The tyre also has more rubber touching the road (larger contact patch), creating more friction — and more heat.

At highway speeds, this heat builds rapidly. If the internal temperature exceeds the tyre's design limits, the rubber and steel components can fail catastrophically — a blowout.

THE STATISTICS

According to the US National Highway Traffic Safety Administration (NHTSA), tyre-related crashes cause approximately 11,000 accidents annually, with 646 fatalities in 2023. Key findings:

• A tyre is 3× more likely to be a crash factor when underinflated by 25%+
• 9% of all crashes involve pre-existing tyre problems
• 45% of SUVs with tyre issues rolled over (vs 25% for other vehicles)
• 75% of roadside flats were preceded by slow leak or underinflation
• Only 19% of drivers properly maintain tyre pressure

Sources: NHTSA TireWise, DOT HS 811 617, National Motor Vehicle Crash Causation Survey

THE GOLDEN RULES

SEASONAL ADJUSTMENTS FOR NZ

Season	Typical Temps	Action
🌸 Spring (Sep–Nov)	10–18°C rising	Set baseline PSI, monitor as temps rise
☀️ Summer (Dec–Feb)	18–30°C	Don't adjust for hot readings — they're normal
🍂 Autumn (Mar–May)	10–20°C falling	Top up as temps drop, especially late autumn
❄️ Winter (Jun–Aug)	0–12°C	Add 2-3 PSI to compensate for cold

WHERE TO FIND YOUR RECOMMENDED PSI

Your vehicle's correct tyre pressure is found in one of these locations (in order of reliability):

1. Driver's door jamb sticker — the most common location
2. Fuel filler flap — some European vehicles
3. Owner's manual — check the specifications section
4. Glove box sticker — older vehicles sometimes

THE PAYOFF

Keeping tyres properly inflated delivers real benefits:

• Up to 8% fuel savings — underinflated tyres waste fuel through rolling resistance
• +4,700 km tyre life — proper inflation means even wear
• Better handling — correct pressure = designed contact patch
• Shorter braking distances — optimal grip in all conditions
• Reduced blowout risk — by an order of magnitude

Source: NHTSA TireWise, nhtsa.gov

THE HIDDEN COST: RUCS & ODOMETER OVER-READ

Here's something most drivers don't realise: underinflated tyres make your odometer tick over faster.

When a tyre is low on pressure, it squishes under load, reducing its effective rolling radius. A smaller radius means more wheel rotations to cover the same distance — and your odometer counts rotations, not actual kilometres. The result? You're recording 2-3% more kms than you've actually travelled.

TOTAL ANNUAL COST OF UNDERINFLATION

Cost Type	How It Adds Up	Annual Waste
🛢️ Fuel (8% worse economy)	15,000km × 8L/100km × $2.80 × 8%	$269
📍 RUCs (phantom kms)	+375km × $0.076/km	$29
🛞 Tyre wear (25% faster)	Replace every 3 yrs vs 4 yrs	$50
TOTAL	Per year, per vehicle	~$350

Over 5 years of vehicle ownership, that's $1,750 wasted — simply because of low tyre pressure. A monthly 2-minute check at the petrol station could save you enough for a nice weekend away.