Low tyre pressure

What happens when tyre pressure is below the norm?

It should be said straight away that dropping tyre pressure by more than 15% is decidedly pointless and dangerous: it brings no further benefit at all, while safety suffers substantially, so let us look at the pros and cons of running low pressure specifically within the 5-15% range (below the norm the car maker specifies).

So, when you lower tyre pressure, roughly the following happens:

1. The tyre’s contact patch with the road surface grows, and it grows thanks to the area of the tyre that is not meant to touch the road at all (the sidewalls). But that only happens where the pressure is substantially below the norm. If you drop the pressure by just 10-15% and the tread is not badly worn, all that changes is how the load is distributed across the tread, so it wears unevenly, that is, tread wear happens faster at the edges. The tyre’s off-road ability improves as a result, because the car’s weight per unit area of the contact patch is reduced. Another consequence linked to the larger contact patch is higher fuel consumption (though, if we are talking about the stated 5-15% pressure drop, you are unlikely to notice any real difference in fuel consumption).

2. The wheel becomes “softer”. With lower pressure, stiffness drops and the tyre absorbs more of the road’s unevenness, passing much smaller loads on to the key parts of the suspension: the arms, ball joints, track rod ends and the shock absorbers’ top mounts. This makes the ride itself far more comfortable over potholes, and braking distance on an uneven road surface gets shorter. There is also a downside: handling and stability on the road suffer. But unless you like to push a car to everything it is capable of, you will not notice these changes.

So, deliberately running tyre pressure, say, 5%, 10% or 15% below what the car maker recommends in poor road conditions, gives you more comfort, a longer service life for the car’s suspension and somewhat better braking on a poor road surface, but the tyres wear faster, you need to drive more carefully and fuel consumption increases slightly. Whether to do this or not is up to you.

Let me stress once more: this article is not a recommendation to act on, because, as noted at the start, the only correct and safe pressure is the one the car maker worked out. So everything you have read above should be treated purely as a personal opinion. Remember that, whatever happens, all responsibility for any consequences of your decision about what pressure to run in your tyres lies with you, and with nobody else.