Why tyre pressure is important



Unless you’re a car enthusiast, chances are you haven’t checked the air-pressure in your tyres recently. I don’t just mean a visual inspection, or doing a ‘kick test’, I’m talking about whipping out a proper tyre pressure gauge, removing the valve stems and measuring the actual air-pressure inside your tyres. When was the last time you did that? Perhaps you’re one of the many people who never have?

Well, here’s your opportunity to make things right.


A visual inspection is not enough.

Now, I’m not here you make you feel bad, but without properly measuring the air pressure in your tyres it is nigh on impossible to figure out whether they’re under-inflated or over-inflated. To demonstrate this, we deflated the 175/65R14 rear tyre on a Honda Jazz hatchback from 32psi (recommended) down to several different levels of under-inflation, so we can see how the tyre changes visually.

Click to view full size

What we found most alarming about this exercise is just how little the tyre changed visually as the pressure dropped. Using the recommended 32psi as a comparison, there was virtually no visual difference when the tyre was deflated to 24psi – that’s almost 30% under the recommended level. Amazingly, the tyre continued to hold its shape right down to around 14psi – at which point the bottom began to look ‘flat’ visually. It progressively got worse down to 8psi, at which point we concluded the test.

So what can we learn from this? Just because the tyres on your car look properly inflated, doesn’t mean they actually are. Failing to properly check the air-pressure in your tyres over extended periods of time mean that your tyres could drop to less than half their recommended air-pressure level before you’ll see a visual difference.

This could easily put the lives of you and your family at risk.


What makes under-inflation dangerous?

Driving a car with one or more under-inflated tyres is a dangerous proposition, because reducing a tyre’s inflation pressure also increases its rolling resistance. If you look at the 8psi tyre above, it is clear that there’s a lot more tyre touching the road surface at any given time, plus the sidewall of the tyre has deformed substantially. This extra rolling resistance and movement means the tyre will not only wear out faster, but it will also run hotter. The increased flexing of the sidewall increases the temperature of the tyre, which in turn increases the risk of a tyre failure and blowout. This risk increases substantially when the car is driven at high speeds, as more friction (and heat) is generated.


Under-inflation will also cause the vehicle’s handling and grip levels to decrease, potentially causing irregular or unpredictable car behaviour in the event of an sudden evasive or emergency manoeuvre. Tyre life decreases roughly 10 percent for every 10 percent it is under-inflated, and the tyre itself will also begin to wear more rapidly on the shoulders, as shown in the graphic above, meaning you’ll be up for a new set of tyres a lot sooner.


Tyre maximum pressure & over-inflation.


I’ve seen quite a few people online asking questions like “My tyre says 65psi maximum pressure, can’t I just inflate it to that level?” The answer to this is Definitely Not!

The maximum pressure specified on your tyre is simply that – the maximum pressure to which the tyre can handle safely – and inflating your tyres to this level essentially means they’ll be running right at the raggedy edge of what they’re been built to handle. That’s a safety margin of zero. It is also vital to understand that as a tyre heats up whilst driving, the air-pressure inside will rise a few additional psi. It isn’t much, but it’d definitely push you beyond the maximum pressure the tyre can handle.

Other downsides to over-inflated tyres are an incredibly hard and uncomfortable ride, a reduction in handling and braking performance, plus excessive wear down the middle of the tyre. So please, don’t over inflate your tyres!


How you can make things right!


There’s two things you’re going to need to check your tyre pressures properly:

  1. A tyre pressure gauge; and
  2. A portable air-compressor.

Both are reasonably cheap and available at all automotive stores. Most air-compressors have their own air-pressure gauges built in, but in my experience these can’t always be trusted – so make sure you get a quality stand-alone tyre pressure gauge too.

Next, you’ll need to locate the tyre pressure placard on your vehicle. Most of the time, these can be found by opening the driver’s door and looking around the lower edges of the opening / door jamb. Depending on the vehicle, it’ll either be a fairly straight forward diagram, or a more complex table like the one shown on the right below.


As you can see, the one on the left is much easier to work out – it shows the air-pressure in kPa, (kgf/cm2) and lastly in psi – the last of which is what you want – so 33psi front, 32 psi rear.

The second example is from a 4WD/SUV vehicle and is more complex, due to the manufacturer listing multiple tyre sizes on the one placard. What you’ll need to do in this situation is check the sidewall of your tyres to ascertain which tyre size the manufacturer fitted to your vehicle, then look it up on the placard to work out your psi pressure. If it was a P185/75SR14 tyre and you ran normal loads in the vehicle, you’d inflate both front and rear tyres to 28psi.


Once you know the specified pressure, grab your tyre pressure gauge and choose one of the tyres on your vehicle to check first. To do this, all you need to do is unscrew the tyre’s valve cap and obtain an accurate reading of the pressure by holding the gauge against the valve. If in doubt, consult the instructions which came with the gauge, as the way they operate can vary.

If the tyre pressure is lower than the recommended figure, connect your portable air-compressor and inflate to the correct level. If your pump has a pressure gauge on it, I would recommend over inflating by 1 or 2PSI, and then using your standalone tyre pressure gauge to bleed the pressure down to the correct level for a more accurate reading. This can be done by holding the pressure gauge lightly against the valve until you can hear air rushing out.

If the tyre pressure is higher than recommended, use the tyre pressure gauge to bleed off some of the pressure until the reading is at the correct level.

Don’t forget to do your spare tyre too!


How often should I check my tyre pressure?

As often as possible! Some car enthusiasts check their tyre pressures weekly, to ensure they’re getting the absolute best out of their tyres – but a more realistic suggestion would be at least once a month.

The reason you need to keep checking is because tyres do lose air-pressure over time, sometimes 1 or 2psi per month, and factors such as temperature can also exacerbate the situation. If you inflate your tyres to their correct level on a hot day, the air-pressure inside the tyre will reduce if the weather suddenly gets cold. Month to month this typically isn’t an issue, but it can be a major factor if you’re entering Winter and haven’t checked your tyre pressures since the Summer.

Checking your tyres monthly will also give you added peace of mind that you’re being responsible and looking after the wellbeing of your family, and that your tyres will be performing at their best when you need them most.

Happy motoring!

Based in Brisbane, Australia, Sean has loved cars his entire life. At 21 he launched the popular 80’s Falcon forum xFalcon.com, then at 24 created TopGearbox.com, one of the most popular Top Gear fansites in the world.