I've overinflated my car tyres. What's the worst that can happen?

August 26, 2012 7:36 AM

I have inflated my car tyres to just under the "max inflate" number, printed on the tyres (they're at 42 PSI, max inflate is 44). I then looked in the vehicle manual, which said that they should be at 33 front, 30-32 back depending on load. Now for my actual question(s): Do I need to go back to the garage and let some air out? What's the worst that could happen?

The car is a 10y old Toyota Yaris, and I think the tyres are the same age as the car. When I attached the pump in the garage they all read very low (in the low 20s) so I guess I should have tried to sort the tyre pressure some time ago. I'm new to this - it is my first ever car, hence the stupid inflation error. I'm going to be driving about 800 miles in the next two weeks.

posted by handee to travel & transportation (28 answers total)

The maximum pressure on the tires is what they are rated up to, NOT the maximum that is appropriate for your car. You should always inflate to the pressure in the manual or on the sticker in the doorfram. Running tires over-inflated will cause uneven wear on the tread (more wear in the centre) and can lead to blow-outs if the tires get hot.

The worst that can happen is a highway blowout, I guess. I'd let some air out for safety. If the car's been driven for a while on under-inflated tires you'll want to check tread wear as well.
posted by Dipsomaniac at 7:47 AM on August 26, 2012 [3 favorites]

Let some air out. The recommended inflation in the vehicle manual is what you should be following. Cold inflated to "MAX PSI" is dangerous: once the tyres get up to operating temperature, they will likely be overpressure as the air inside heats up and thus increases the pressure. Even if they don't blow out (not real likely but much more possible than at normal inflations), the ride will be very harsh and jarring which is not good for the suspension or for you. Also, the tyre contact patch will be smaller, and they will wear in the middle, accelerating problems.

So, follow the manual's guidelines, for safety and comfort.
posted by seanmpuckett at 7:48 AM on August 26, 2012

When you drive, the constant flexing of the tires heats them up, causing the air pressure to rise; this is why you should check their pressure when the car has been sitting for a while. It also means that if they're at the max pressure cold, they'll be over the max when driven for a while. Overinflation will give you slightly better fuel mileage but will also give you somewhat worse traction, particularly in the rain. If these are old tires there probably isn't much tread; I'd reduce the pressure to what is recommended & then check periodically.

This is besides what Dipsomaniac said, which is also true.
posted by mr vino at 7:49 AM on August 26, 2012

Yeah, you should let some out. If you don't want to go back to the garage, you can buy a pressure gauge (not expensive, and you should probably have one anyway) and just let air out of the tires, checking pressure often, until they're inflated at the right pressure. You'll hear the air coming out if you press the gauge or any other pointy object (ballpoint pen, etc) into the valve.
posted by geegollygosh at 7:50 AM on August 26, 2012 [1 favorite]

The worst that can happen: you die taking others with you.

How? Over inflation reduces the contact patch between your car and the road, making your car's handling worse. So an emergency maneuver fails, and you smack into some else's car and kill some of them and then die yourself.

Other more probable bad things:

*Your ride will be very harsh. Bad if you like comfort, good if you're trying to dislodge a kidney stone.

*Your tires will wear very fast in the center, seriously reducing your tires' lifespan.

*A BLOWOUT. You might has one.
posted by ROU_Xenophobe at 7:51 AM on August 26, 2012 [4 favorites]

Yes, get the tires back down to the pressure recommended in the manual.

Apologies if you already know this, but just covering my bases:
You can let your own air out, with either a cheap tire pressure gauge (the chrome-looking tube ones) or if you are in a real pickle, your keys. With the tire pressure gauge, use the small lump on the back to press into the valve stem, or you can do the same thing with the tip of a key. Do it for 5-10 seconds at a time, and check the pressure, repeat until you're in the range.
posted by deezil at 7:51 AM on August 26, 2012

Yes, please let some air out. Over inflating tires reduces the size of the contact patch that the tire has with the ground, thus you are reducing the grip the tires have on the road. Less grip can encourage accidents and dangerous unpredictable handling.
posted by neveroddoreven at 7:57 AM on August 26, 2012

I don't know where you are, but in the US most normal gas stations have air compressors with tire gauges on them so that you can accurately inflate/deflate your tires. Just stop at the closest one the next time you drive your car and take care of it, it will take a few minutes at most. (The gauge will work for free, but you should bring a couple of quarters in case you deflate too much and need to add air to your tires.)
posted by anaelith at 7:58 AM on August 26, 2012

Many thanks - I've got a track pump for my bike, which has a built in pressure gauge, and I've just used that to take them down to 30-35 (it's not a very detailed gauge so I couldn't really be more accurate). I'll stop next time I pass a garage and do them all properly. Thanks again!
posted by handee at 8:10 AM on August 26, 2012

It's probably a good idea to get in the habit of regularly checking your tyre pressure and to maintain it at recommended pressure because it maximises tyre life and fuel economy as well as your safety.
posted by koahiatamadl at 8:26 AM on August 26, 2012

Yes, you should let the extra air out. But it really isn't as dangerous as people here are making it out to be. Underinflation is FAR more dangerous. You are much better off with a tire that is inflated on the high end of the specifications, than with what you had- a tire that was inflated far under the minimum acceptable inflation. Underinflated tires flex more and heat up more and this causes blowouts. That was part of the problem with the Ford Explorers that flipped over- they were telling owners to only fill the tires up to 26 PSI. Which, over time, caused the tires to weaken and eventually fail.

Even if you are at the maximum for that tire, you are still within specifications. A tire, especially a steel-belted radial made for a passenger vehicle, isn't going to change its contact patch all that much while it is still within specifications. And the car will still exert the same amount of pressure on the ground, since its weight stays the same.

Cold inflated to "MAX PSI" is dangerous: once the tyres get up to operating temperature, they will likely be overpressure as the air inside heats up and thus increases the pressure.

This is incorrect. The MAX PSI rating is measured at room temperature.
posted by gjc at 8:36 AM on August 26, 2012 [1 favorite]

The max tire pressure on the side of the tire IS the max cold pressure. This pressure takes into account additional pressure gain from heating, which usually isnt much. To get an accurate reading you need to measure them cold-which means not driven on(much) and not parked in the sun either. It is perfectly safe to run the tires at 44 psi gauge pressure as long as the measurement is done cold. In a lot of conditions you will actually get more grip on fully inflated tires than on partially inflated tires. It is especiaaly helpful in wet or snowy conditions. The car will also have better steering response and give you more 'feedback' at the limit (not a big deal in a yaris however) and a little better gas mileage. The bad side is the fully inflated tires will wear out the center faster and ride really harshly. The engineers at toyota generally know what they are doing so, for the average driver, just go with the recommended inflation pressure, it is the best compromise for your car. Dont forget you also have a spare to check, and ALWAYS inflate hide a spare donuts to the max setting listed in the sidewall.
posted by bartonlong at 8:40 AM on August 26, 2012 [1 favorite]

But it really isn't as dangerous as people here are making it out to be.

In terms of handling it certainly is, particularly in the wet. If you over-inflate your tyres when cold and then load your car to a heavy weight (ie with passengers and luggage) it can get very dangerous very fast - over heating tyres and poor contact patch means very bad grip levels.

Under inflated tyres are also extremely ba for handling, and this is more likely to create issues - an under inflated tyre will work (and potentially damage) the sidewall, which if you then inflate to maximum pressure could create a worst case situation. But you'd have to have been running on them for a while in the low 20's at the most to get this kind of damage, but it's a factor.

It is pretty unlikely that you will get them hot enough to blow out (although if they are 10 year old tyres then this rises very fast as a possibility for separate reason). If your tyres are 10 years old, get new ones immediately. Rubber degrades in UV light and 10 years is WAY too long. They sidewalls are likely structurally compromised and high speed driving in particular is a high risk action on tyres that old. Replace them with good quality ones.

This is incorrect. The MAX PSI rating is measured at room temperature.

This is true. All tyre pressure and ratings are given at the same temperature and all measurements should be taken at or around 20 deg C and modified accordingly if the temperature varies. So set them lower if it is colder, higher if it is hotter, although it's not a big difference. It's about 0.1 PSI (very roughly for a ballpark number) for 1 degree for an average tyre, so setting at 30 degrees you'd set 1psi higher, setting at 0 degrees C you'd set 2 psi lower.
posted by Brockles at 9:25 AM on August 26, 2012 [2 favorites]

I would consider 10 year old tires, regardless of wear, in need of replacement.
posted by spitbull at 9:35 AM on August 26, 2012 [3 favorites]

Buy a proper automotive tire pressure gauge and keep it in your car. Depending on the one attached to garage pumps isn't a great idea since you don't know how well they're maintained.

And at 10 years old, you probably need new tires.
posted by chairface at 9:37 AM on August 26, 2012

The only thing I came in here to chime in on is that 10 years is a long time for the same set of tires.
posted by radioamy at 9:57 AM on August 26, 2012

OK - it passed its MOT test 3 months ago and the tyres were fine then. It's only done 35K miles which is not much for a 10 year old car, and spent much of its life under a carport. I'll get someone who knows about cars to take a look at them though.
posted by handee at 10:02 AM on August 26, 2012

MOT tests do not consider age of rubber, only depth and spread of tread. They have no way to measure sidewall integrity or UV degradation. I am someone that knows about cars and your tyres needed replacing if they are 10 years old.

The mileage is irrelevant - that only affects the physical tread. The UV degradation (the quality of the rubber) is the issue and this is not limited to damage through direct sunlight so being under an open roof won't help too much. Besides 35,000 miles is a long time to be out of the carport. Don't look for justification to keep tyres that are well past their expected life.
posted by Brockles at 10:14 AM on August 26, 2012 [3 favorites]

Racer/Autocrosser/Car guy here.

A couple PSI over the recommended pressure isn't a problem. We race with some over inflation, and the tires are subject to increased heat and driving forces. I'm talking high 30's, but sometimes low 40s.

In fact, I usually run at about 38 PSI on a daily basis and have never seen abnormal wear. Modern tires are quite rigid, and don't deform as much as older tires.

For best overall performance and comfort, run the factory recommended pressure. It's almost universally 32 psi for passenger cars.

"In terms of handling it certainly is (dangerous), particularly in the wet"

This is incorrect. Higher pressures reduce hydroplaning and increase traction in snow (to a point). But I'm getting nerdy about it.

You are already ahead of most drivers by paying attention to tire pressure. Just use 32 psi and check them every few weeks.
posted by colinshark at 10:29 AM on August 26, 2012 [1 favorite]

Agree that the tires need replacing at 10 years old. Rubber degrades in several ways over time, and none of them are good. They can also be dangerous to fill, because a blowout while standing next to your tire can be deadly. It's crazy, but it happens.
posted by colinshark at 10:33 AM on August 26, 2012

The usual rule of thumb for tires is 40K miles, but rubber deteriorates. UV is one factor, but so is simple oxidation. Ever try to use a really old rubber band from the back of the drawer? Right. I, personally, would not feel safe on 10-year-old tires, certainly not at highway speeds.
posted by dhartung at 10:35 AM on August 26, 2012

This is incorrect. Higher pressures reduce hydroplaning and increase traction in snow (to a point)

It is not incorrect, because I am referring to over-inflation whereas you are describing higher normal pressures, it seems. Higher pressures within the normal range of pressure can reduce hydroplaning over a softer (pressured) tyre (also within the normal range), provided the tread form of the tyre is maintained as per design. Essentially it can provide rigidity to allow the designed tread form to move water more effectively. However, this is only within a certain range of the normal operating pressure. Worn tyres reduce this effect, old and degraded tyres do not show so much of this effect.

Over inflated tyres (above the normal range of around 30-36 psi) have a progressively reduced contact patch. This reduces grip. This reduces the ability of the tread form to reject the water on the road surface because less of it is touching the road and so will increase the chance of aquaplaning.

We race with some over inflation, and the tires are subject to increased heat and driving forces. I'm talking high 30's, but sometimes low 40s.

I've run many dozens of road and race cars on road tyres at similar pressure on race tracks. You will see much increased tyre wear for the increased grip. In addition, running higher pressures on the road as you do will decrease the car's ability to maintain consistent grip over road irregularities and as a side effect reduce ride quality. It is a sub-optimal pressure for regular road driving, particularly in temperatures other than those that means the road surface is above (say) 30 degrees C (86F).
posted by Brockles at 10:50 AM on August 26, 2012

seanmpuckett writes "Cold inflated to 'MAX PSI' is dangerous:"

It's important to note that while running max tire rating inflation pressures can be dangerous it's not because of the risk of the tire blowing out but because of the effects maximum inflation can have on handling and braking.

For example trailer tires should almost always be inflated to their maximum side wall pressure. And it is fairly routine for even light trucks to have their rear tires inflated to their maximum tire rating when hauling loads. In fact the load rating of a tire is at maximum inflation pressure. At lower pressures the load rating is reduced.
posted by Mitheral at 12:42 PM on August 26, 2012

OK, so I may well need me some new tyres! Follow up question for anyone still reading: Is there any reason not to go for the cheapest? Looking at kwik fit, I can get 4 tyres with fitting and environmental disposal for about 170 quid, or I can pay much more. Cheap ones are: Arrowspeed.
posted by handee at 12:43 PM on August 26, 2012

Answering my own supplementary question, just in case any future askers are similarly clueless about cars - they seem to be OK.
posted by handee at 1:41 PM on August 26, 2012

they seem to be OK.

The cheapest are never the best. This is certainly the case with tyres. Those seem to do ok for the price, but that isn't the same as 'are they good enough'. Tyres are the most critical component on your car for safety. Don't scrimp on them - get a reputable brand and head for somewhere in the middle of the price range if you are trying to save. I would never buy the cheapest tyres I could get.

In addition, if you do such little mileage as it suggests you do if your car has done 35k in 10 years, then consider buying a part worn tyre of the better brands. There should be something in your area that provides these. You get a lot of Continental part worn tyres, for instance, in the UK (because of the more aggressive minimum tread depth in Europe - Germany in particular). Continental are really good tyres so this may be your best bet.
posted by Brockles at 1:52 PM on August 26, 2012

You might want to get a car tire pressure gauge-- bike tires are usually run at much higher pressures than car tires, so you can't necessarily count on a bike pump gauge in a car tire pressure range. Tire pressure gauges are quite cheap.
posted by akgerber at 7:54 PM on August 26, 2012

A tire inflated below the maximum but above the minimum is by definition within the normal range of pressures.
posted by gjc at 5:04 PM on August 27, 2012