Tyre pressures

The tyre pressures you need are specific to your *car*, not the tyres. Use the figures on the door edge. The value the tyre fitters used will be unlikely to be correct IME...

The pressure should be checked before the car has been driven. Even a short distance can make a significant difference to the pressure.

The pressures will be higher if the car is fully loaded. A short trip into town at low speeds with the car full is not going to matter, but if you usually travel with the car full you may want to err on the side of the higher value.

Note that garage tyre inflators are not very accurate; for optimum safety and best tyre life get a separate gauge.

HTH

Chris

I would also add that regualr checks of the pressure vastly improves tyre life, mpg, and handling characteristics of the car, not so much day to day (you get used to the car) but more so how the car behaves in an emergency- could be life or death.

On a personal note, I always find I need to run somewhere between 'normal' and 'fully laden' pressures in the front tyres (i.e. on the firm side) but often 'normal' pressure in the rears, to create the handling balance I prefer.

I also consistently get somewhere in the region of 20k + miles out of set of tyres despite my fairly keen driving style, purely through running the right pressures, tracking that's spot on and regular rotation.

Tim..

Thanks to you both for the help, I'm still having trouble getting my head round the fact that the correct tyre pressures are specific to the car not the tyres!

I'd have thought that a tyre has a specific "correct" pressure regardless of the car it is attached to, that a tyre has its correct operating "shape"at a specific pressure (ie too high a pressure would cause wear at the centre of a tread and too low wear at the sides/edges)

If for example a tyre is correctly inflated at 30psi and it is attached to a bentley, the weight of the car is going to increase internal pressure and you'd have to let some air out to get back to the optimum. If you put the same tyre on a fiesta (if it'd fit) the lower weight would require less air to be removed.

Am I just having a "dense" moment? Or am I looking at this from the wrong angle? Or both!

BP

I'm afraid the latter would seem likely :-)

Chris

Indeed. A recent local single-car accident resulted in the loss of the lives of both occupants. The car left the road on a gentle bend in good weather conditions during daylight. The driver was a lady in her late '30's; the passenger her mother. The road has a 50mph limit. The bend I would estimate I could easily get round at 70+.

The conclusion at the inquest was that the accident was solely attributable to under-inflated rear tyres.

Chris

IME recommended tyre pressures for non OE tyres are not necessarily ideal pressures. Tyre pressures within a few pounds is not that critical, as long as tyre wear is even across the tread. My advice would be to simply use the recommended pressures as a starting point, varying them by a pound or two in steps either way, until you arrive at a pressure which appears to suit the car, tyres, and your style of driving. If you do choose to experiment, buy a tyre pressure guage and always use it. You can't rely on garage guages to be accurate. If you're own guage is wrong it's not so important, as it will still give consistant pressure comparisons for any changes you may make. Mike.

I think you don't see a very big difference in tyre pressure related to load. Try checking the tyre pressure on a wheel that's jacked up, then let the car down and check it again. I don't think it moves much.

For aeroplanes the difference is a bit bigger.

But the shape that the tyre adopts does depend on the weight on it. Basically if it's 30 psi and supporting 30 pounds, then the contact patch will be about an inch in area. So double the weight and the tyres will squash down more. So for that reason you'd expect the desired pressure to depend on the car.

The strength of the tyre casing is what determines the maximum pressure it can hold, and that depends on the tyre. But car tyres operate some way below the pressure at which they'd explode or blow off the rim. I know this because they tried it on a third rate Top Gear clone I saw on Sky Three, the tyre took about 250psi before it blew up, smashing the rim in the process.

You can stretch your fuel economy a bit by running at 40 psi or so.

Have a look at

But can you trust a five quid job from Halfords to be any better?

Ian

A few years ago, I had access to a lot of test and calibration equipment. I set up a rig to evaluate tyre pressure gauges. I had a couple of my own, including an expensive "professional" bourdon tube analogue one. I bought another two, an average price one and a very cheap one from Halfrauds. I borrowed others from work colleagues to give me a total of ten.

None of them were inaccurate enough to be dangerous; the surprise was the degree of accuracy of the el cheapo one! It was absolutely spot on over the range I tested, and repeatably so. It was such a low price thing that the battery was non-replaceable. Oh, if I recall correctly, it was less than five quid...

Chris

Note that if 40 psi is significantly different from the manufacturer's value for your vehicle, you would be in breach of UK law however.

Chris

You snipped the relevant part of my post. Even if the error is as much as a pound or two on a Halfords one, it doesn't really matter as long as the error is consistant. Such a guage will still allow you to 'adjust' the tyre pressures accurately. Something you can't do if you use different garage airline guages which could vary from garage to garage.

I have actually got a Halfords guage. Cost £9.99. Checking it against more accurate guages at work, I would say it's accurate within half a pound. Mike.

It depends on what car, what size tyre and how much weight is in the car. The tyre will explode at 300PSI so don't go overfilling it. Too much air will make less of the tyre have contact with the road and will make the car faster in a straight line, but crap at corners. Too little air and you will damage the tyre. I can't believe you have never ever checked your tyre pressures in all the time you have owned the car. You sound a bit thick to me! You are saying there is a sticker telling you what tyre pressures to use, then asking how to find out. The simple answer is use your common sense.

Lol, Hi "Trev", you sound a bit thick to me too!!

Where did I say I have never checked the tyre pressures on my car? Did you bother to read what I said in my original post?

What I said was that I had just had two new tyres fitted and I wondered whether the label on the door was relevent to these or was it specific to the original factory fitted tyres. I was wondering if each make of tyre has its own optimum pressure or are all makes designed to "run" at roughly the same pressure!

I regularly check the tyre pressures on the car, I was just unsure whether the pressure on the door was relevant to the new tyres!

Does that help explain it to you?,

I always do use common sense, it seem you have no sense at all, common or otherwise !

BP

(I typed this slowly btw as I'm guessing you cant read very fast!!)

If they're the same size then the door cards probably right, it'll be way closer than the max pressure marked on the sidewall. After that , unless you want to buy a pyrometer, you can judge by the wear on the tyre.

I don't think so.

I don't understand what you mean by "adjust" here? Why does an inaccurate personal gauge allow you to "adjust" better than an inaccurate garage gauge?

Ian

Thanks for that. I wonder if they are all that good, or you just got lucky?

Ian

Yes, I've wondered that myself. I didn't have the budget to buy a batch of them!

Chris

Because the /same/ inaccurate guage will be repeatable. Different ones will have different errors.