The Real Cost of Driving on Under-Inflated Tyres
A practical guide to under inflated tyres for UK drivers, from the CarPal mobile tyre team.
What happens inside an under-inflated tyre
With too little air, the tyre flexes more with every rotation, particularly in the sidewall. That flexing generates heat, and heat is what destroys tyres.
At low speeds it is merely wasteful. At sustained motorway speeds, a badly under-inflated tyre can build enough heat for the internal structure to separate, which is what a blowout actually is.
The wear pattern
Under-inflation loads the shoulders of the tyre rather than the centre, producing wear on both outer edges while the middle of the tread still looks reasonable.
That is the mirror image of over-inflation wear, and it is equally diagnostic. Both edges worn means pressure; one edge worn means alignment.
The running costs
Under-inflated tyres have higher rolling resistance, so the engine works harder and fuel consumption rises. The effect is small per tyre but constant.
They also wear out faster, so you replace them sooner. Between fuel and tyre life, running consistently low costs real money over a year.
Handling and braking
The car feels vaguer, steering response slows, and the tyre is more prone to rolling onto its shoulder in corners. Braking distances increase, especially in the wet.
None of this is dramatic at 20 PSI below spec on a supermarket run. All of it matters at 70mph in rain.
Frequently asked questions
What happens if you drive on under-inflated tyres?
They flex and overheat, wear on both outer edges, use more fuel, and in severe cases can fail at speed.
How much under-inflation is dangerous?
TPMS typically warns at around 25% below spec. Anything approaching that should be corrected before a motorway journey.
Do under-inflated tyres use more fuel?
Yes. Higher rolling resistance means the engine works harder to maintain the same speed.
What wear pattern does under-inflation cause?
Wear on both outer shoulders of the tread with the centre relatively less worn.
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