Car tyre pressures

[JonnyG]

You will never have the same pressure in front and rear tyres as the load distribution is different. That's why you inflate the rear tyres more because putting people in the back seats and loading the boot puts more load on the back axle. Every manufacturer's handbooks I've ever seen always give pressures for nominal load and max load. I rest my case.

sorry you are mistaken again ! many cars have the same 'recommended pressures in the front and the back under what is deemed 'normal' load..
sigh. so sorry you dont understand what i was saying have a look at your rear tyres and see what the max tyre pressure is for that tyre.as is written on its sidewall..now look at your handbook, your hand book will not have the same max tyre pressure as is written on the tyre! the handbook is a recommendation and nothing else!!!!!!!!!!!!

 

[JonnyG]

Part of the problem is we have been brainwashed to think that DIGITAL systems means they must be accurate. Frankly that is not always true.

Any decent piece of equipment will have a manufactures specification that tells you how accurate it should be, for example the digital devices will usually have a basic +/- accuracy followed by number of significant digits.

But even given that information, unless the device has been calibrated to identify the actual inaccuracy of the instrument, you cannot trust digital devices any more than older analog devices, and the OP's experience demonstrates this.

So how can you really know if your gauge is accurate, the only way is to get it calibrated, but being realistic and in the context of checking tyre pressures, and accuracy of about +/- 1psi or 0.1Bar is probably more than adequate.

As forecourt tyre pressure systems are supplying a commodity, they should be calibrated so they will probably be more accurate than any domestic gauge.

What is as important as getting the right pressure is to get the same pressure in the tyres on each axle.

But what is the right pressure? Car manufactures work with tyre manufactures to select the most appropriate tyres for cars they produce. As a consequence I cannot see how Jonny thinks they don't know. So be guided by the car manufactures pressure recommendations.

Obviously if you change the size of tyres, then some recalculation of pressure may be necessary.

oh dear prof! what rubbish you have come out with for once! thought i was reading one of those non describt hand outs!

' " Car manufactures work with tyre manufactures to select the most appropriate tyres for cars they produce." hmm really? what rubbish they select the most appriprate tyre from whoever they have a contract to supply them tyres!.. totally different senario! and not nessacarily the best tyre on the market.which should be obvious given all the trye manufactuers out there and lets be clear companies like michelin isnt going to go over board inventing a new tyre specially for a ford focus!!!

and then theres this one!! i mean you almost state the obvious, but not how to do it?
"Obviously if you change the size of tyres, then some recalculation of pressure may be necessary."

you think! pray tell me how is one going to do that!!! what i mean is recalculate as you advise,sorry it might be necessary.to recaculate!
please do tell me how you would do that! who would you ask? surely you must have a formula!.. No? So what are you acutally saying here Prof? because you dont know much about tyres no one else could??? please bear in mind any car manufacturers recommendations to psi will be based on those by the trye company thats providing the tyres and i repeat as a guide!

read this properly tyre pressures by car manufacturers state " recomended " not legal .strange how you can understand the 85% rule is only a recomendation yet give you two pence worth on tyres and ignore that it too is a recomendation and nothing else a guidline for those beginners. in tyres
oh and as a foot note i find it strange when people harp on about the correct tyre pressure and at the same time harp on about the inacurate tools for measurement! which surely means most people are driving around happily so on the "not recommended by the car manufacturer" tyre pressures!!!!!!!!!!!
http://toyotires.com/tires-101/load-and-inflation-tables this is what i use and strangely it gives me proper not recomendations

 

[ProfJohnL]

Do you feel better now after your rant! Why don't you calm down take a breath and carefully read what has been written, and stop jumping to inaccurate conclusions.

I have never claimed to be an expert on tyres, but in reality you do not know the extent of my knowledge or experience, and I do not know yours, which is always going to be issue with anonymous forums. So why should readers believe either of us?

Hopefully coherent reasoned discussion will give readers more chance to understand the issues and ultimately come to a considered choice, but as I always suggest in my signature space, readers should verify any information with professionals before acting on it.

Towing ratio recommendation of 85% is a unilateral figure with no technical justification or link to the outfit. Manufacturers tyre pressures may be recommendations, but they do have credence as they will have taken into account the range of loads the vehicle is specified to carry for the quoted tyre size.

Interestingly, the opening paragraph of the tyre companies table listing states
"For inflation pressure recommendations for the original equipment (OE)size, refer to the tire information placard (T.I.P.) or owner’s manual." The TIP is usually found attached to the vehicle.

I never stated the car manufacture selected the "best" tyre, only that it will be appropriate. What defines best is difficult, as that may be down to the drivers preference for balance between economy, grip, and cost etc.

It is my experience that Manufactures often work with suppliers to identify the optimum product for the job in hand. In this case the tyre size and rating for the vehicle. If they didn't the car would not be a roadworthy design and could not type approved.

I correctly pointed that if the size of wheel/tyre are changed from OEM standard, it may be necessary to recalculate the required pressure. I was only pointing out the necessity, I did not state I had a method. However your reference to tyre pressure tables is actually a method of calculating (as the tables are generated by doing calculations).

It is probably true that many drivers have tyres not inflated to the pressure they think they are due to the inaccuracy of most domestic measurement devices. However I did give what are acceptable pressure reading errors from commercial tyre inflation systems.

 

[JonnyG]

hi prof.. i was amazed how easy you took the notion its in the handbook so follow it mentality, would have hoped you would have done some proper research first,that way seeing where i was coming from.afterall most of the time thats what we expect to hear from you info based on proper research...
I do like reading the towcar of the year stuff and indeed peoples responces to the test results so much on this site is technically based with regards legal and safety wise. and yet when tyres are discussed its like who gives a darn there only peices of rubber ,err let see what the untechnical book says!
In fact when TCOTY .data is ou,t so much is said about how practical the test is set up to make all things equal, i normally scoff when i read that as i wonder as they set up ever vehicle to the recommended 85% rule do they then just open up the non scientific handbook for tyre presures or do they go to a weighbridge measure the front axle weight of the car, then the rear and have someone
on site who can speciffy exactly what the fronts and rears tyre psi should be set at...doubt that at yet its these four small patches that play as big a part in how a car or car trailer handles as anything else!
4 psi can make a difference .in braking distances or how a vehicle handles in any given situation and towing has to be high up on that and yet if i follow things correctly most just take the recommended
figures for a fully loaded car!! err that could mean over inflated more likely to make the tyre skip less grip..do i need to go on!!!!
the handbook recommendations are normally on the safe side afterall your fully loaded car could weigh more by a 100 kg than mine or vice versa 100kg across the back axle
is 2 or more psi in tyre pressure....too low maywell lead to overheating tyres but tell me do you actually know what too low is in relation to tyre pressure? against load rating. too high and like i
said forget ridequality handling of a caravan could be effected......like i said i love the TCOTY awards and they go to great pains to do G test and slaloms test for stability all very technical stuff except i would believe having the tyres pumped up to the right psi given the loading of course i could be wrong on that score but have never seen it mentioned..could make a not so stable car a near class winner......although ok doubtful......

my mondeo "recommends" for my tyres 205/55/16...2.3F 2.1 R normal load.....2.4F 2.8R full load....but ford is one of the better ones it also recommends 2.5F 2.4R normal load and 2.7F ..3.0R full load when travelling above 100mph ! and as high as 3.1 bar when travelling at 140 mph!!!!!!
hmm where does normal load become full load? what about a psi for somewhere in between full and normal load, after all there is a difference on the rear of 0.7 bar thats a 10psi ! difference are you suggestioning 10 psi wouldnt make a difference in handling braking one way or another?

So as you can see ford recommend 2.3 bar or 34 psi for my cars front tyres, normal load ..hmmm and yet the front of my car with me and the wife in it weights in at 880 kg......which is 440kg or 970 Lbs on each of the front tyres which is 28 psi per front tyre and not 34 psi as recommended.. which is 6 psi higher than i need....
i run them at 30 psi daily.....!! especially at this time of year to and fro to work 6 miles..

I find it soo strange that one would trust non experts, hey i'm not one either but nor is kwick fit and the likes, why not ask them next time you change tyres what they recommend for tyre pressures
and ask them to explain why, at best you'll get a non scientific answer of "it works for me" or " i find they work best at ** psi/bar" now isnt that all scientific and easy to follow......

 

[Surfer]

I would agree that digital meters can be very suspect and need to be calibrated annually. I think a analogue meter would be more reliable as it is not subject to various fluctuations in conditions. For example a digitla meter will suffer from damp or moisture whereas a analogue meter will not suffer as much.

 

[JonnyG]

hey prof i am not nit picking but.....

"It is my experience that Manufactures often work with suppliers to identify the optimum product for the job in hand. In this case the tyre size and rating for the vehicle. If they didn't the car would not be a roadworthy design and could not type approved"

Prof that is such a misleading statement or rather the last part of it is...i might over simplyfy things here but speed ratings and load [index load] thats it what else is there, it nothing thats goona effect roadworthy, design or indeed type aproval..... not where mainstream cars are concerned. i mean do car manufacurers have to work with every tyre manufacurers for type approval? no.
and indeed of course they work with tyre manufacturers but not how you would have it come across,otherwise wee'd have a 1000 different spec tyres out there, which we dont....
infact do you read car mags, well if you dont then in the main car manufacturers standard fitment tyres are not usually the winners in test carried out with after market supplied tyres, of course a lot of it is subjective but i am merely making the piont i dont agree your statement carries any validity...and nothing else

 

[Royston]

Looking at my ford handbook, tyres mentioned 205/55/R16 , the normal load and full load pressures are the same at up to 100 mph as they are at continuos speed in excess of 100mph.
Surely the code written on the tyre wall manufactures of tyres must adere to this specification, cannot understand why it has been mentioned that different manufacures the air preessure will differ even though the same code is wriiten on the tyre sidewall.
Getting back to originally comments , the tyre pressures recomended by Ford are spot on, since I first posted i have towed with my mondeo , on windy and normal road conditions the car behaved as it was driving solo ,
Royston

 

[otherclive]

You will never have the same pressure in front and rear tyres as the load distribution is different. That's why you inflate the rear tyres more because putting people in the back seats and loading the boot puts more load on the back axle. Every manufacturer's handbooks I've ever seen always give pressures for nominal load and max load. I rest my case.

not correct. My wife's Note has 34 in the front and 31 rear for up to 3 passengers and luggage. My Xc70 can have anywhere between 33to 38 all round depending on load and speed. And Volvo have the 38psi Eco setting for just general use as well as loaded high speed cruising. My Pajero is 28 front 26 rear for almost everything but my old Sorento was 37 all round for everything. So front is not necessarily lower than rears. I tend to go by the makers recommendations and modify if needed for ride and wear characteristics. It's not a science but something each driver should develop. If I tow with the Pajero I put the rears higher as they are large flexible tyres on 15 inch rims with a raised compliant suspension. Increasing the pressure stiffens the car laterally so less sway.

 

[ProfJohnL]

OK Jonny,

I'm guilty of not having done extensive research in to the accuracy of handbook recommendations for tyre pressures.

It seems from your postings that you contend that all car manufactures systematically misrepresent the correct tyre pressures for their cars. That is a major accusation, which Im surprised has not been challenged in a more robust manner.

Now does that seem reasonable? I don't think so. I am certain the tyre manufacturers would have been sueing the car manufactures left right and center if the published information was wrong.

So what I have done is to consider the probability that manufacturers would go to such extremes of producing a handbook with inaccurate or even unsafe information and allow it to be perpetuated, and repeated ever since.

These days, product hand books are not normally produced by the sales department so the wild advertising claims should not appear in them. They are normally produced from the design department, in conjunction with the various task teams involved in the project. In most manufactures use quite rigorous creation, proofreading, reviewing and editing processes. Documentation does not go to print until various departments have verified and validated the information it contains. So the chances for significant errors such as tyre pressure data is highly unlikely.

But even with the best will in the world sometimes errors will get through. But as soon as the error is pointed out, the the base document would be amended and the revisions would appear in the next issue.

I cannot conceive all the car manufacturers are systematicaly involved in publishing inappropriate tyre pressure information.

You clearly do not understand my perception of the importance of tyres. I cannot predict other peoples interest in the subject, but if car manufacturers were so remiss with their specifications, i'm certain we would have been advised by Which, What Car, and other specialist magazines and other authoritative sources to use table like those published by Toyo.

I'm not going to perpetuate this line of enquirery, suffice to say that following the manufacturers specifications is safe.

 

[Lutz]

The tyre pressures are not only determined by load and speed, but also the way the car handles. For example, if a car is liable to oversteer, the manufacturer will specify a slightly lower tyre pressure at the front (or higher at the rear). Vice versa applies if the car understeers. The car's handling characteristics can be adjusted accordingly to suit.

 

[JonnyG]

OK Jonny,

I'm guilty of not having done extensive research in to the accuracy of handbook recommendations for tyre pressures.

It seems from your postings that you contend that all car manufactures systematically misrepresent the correct tyre pressures for their cars. That is a major accusation, which Im surprised has not been challenged in a more robust manner.

Now does that seem reasonable? I don't think so. I am certain the tyre manufacturers would have been sueing the car manufactures left right and center if the published information was wrong.

So what I have done is to consider the probability that manufacturers would go to such extremes of producing a handbook with inaccurate or even unsafe information and allow it to be perpetuated, and repeated ever since.

These days, product hand books are not normally produced by the sales department so the wild advertising claims should not appear in them. They are normally produced from the design department, in conjunction with the various task teams involved in the project. In most manufactures use quite rigorous creation, proofreading, reviewing and editing processes. Documentation does not go to print until various departments have verified and validated the information it contains. So the chances for significant errors such as tyre pressure data is highly unlikely.

But even with the best will in the world sometimes errors will get through. But as soon as the error is pointed out, the the base document would be amended and the revisions would appear in the next issue.

I cannot conceive all the car manufacturers are systematicaly involved in publishing inappropriate tyre pressure information.

You clearly do not understand my perception of the importance of tyres. I cannot predict other peoples interest in the subject, but if car manufacturers were so remiss with their specifications, i'm certain we would have been advised by Which, What Car, and other specialist magazines and other authoritative sources to use table like those published by Toyo.

I'm not going to perpetuate this line of enquirery, suffice to say that following the manufacturers specifications is safe.

hi prof thats what i would call twisting my words.... not once did i use the word "inappropriate" how does one get from recommended or guide to inappropriate!!!! not once have i said they are unsafe either! "recommended" is the word they use and not legal or safe! and how can someone or people who fully understand the difference that say 5kg can make on a towball to the handling of their outfit not understand how 3 psi eitherway in their tyres can do the same thing. ie make it better safe! Infact if one reads what Lutz has written which is correct and then take a step back how can anyone accept with blind faith that two tyre pressure settings "normal load and full loaded" be the absolute best settings that would apply across the varying weight distrubutions a car could have!
strange that i gave a scientific website explaining things, i could give you more websites! and here you are Prof dismissing it and relying on blind faith! and twisting of my words! and not coming back with one scientific fact either!
if one reads what Lutz has stated and accepts that different tyre makers tyres act slightly different and they do then what makes you think a Michellin tyre will behave identical to a goodyear at the same tyre pressure? of course that doesnt make anyone of them unsafe but merely one will be better in that instance? which makes tyre pressure even more tricky to determin their BEST safest setting!....a recommedation cannot be the best in all varying instances.....

"I'm not going to perpetuate this line of enquirery, suffice to say that following the manufacturers specifications is safe."
well its not unsafe thats for sure i'd use the words acceptable and adequent.. hey if adequent is good enough for you thats fine, me i want as safe as possible and the recommended tyre pressures cannot be as safe as possible totally across the board! do the maths prof....and dont twist my words for effect either.......

 

[otherclive]

I would venture to suggest that 99% of motorists are not blessed with your analytical skills. So many don't even bother to check tyre pressure in weeks so for those that do going by the manufacturers recommendations must help in maintains their safety and the safety of other road users. Better is the enemy of the good enough. Adm Gorshkov

 

[Gafferbill]

The important point to remember when towing is to have adequate tyre pressure........too low a pressure on the rear tyres is the enemy.
You do not want over flexing side walls due to increased load which can happen even if you have inflated your rear tyres to the manufacturers towing recommendations.
You can safely increase tyre pressures up to the maximum for the tyre to combat this.

For example my 4x4 has normal tyre pressures of 26psi all round and the handbook recommends an increase on the rears to 29psi for towing.
I have found 29psi to be totally inadequate for my 100kgs noseweight and rear loading in the boot area so I use 36psi which is fine.
The tyres have a limit of 44psi.

 

[JonnyG]

The important point to remember when towing is to have adequate tyre pressure........too low a pressure on the rear tyres is the enemy.
You do not want over flexing side walls due to increased load which can happen even if you have inflated your rear tyres to the manufacturers towing recommendations.
You can safely increase tyre pressures up to the maximum for the tyre to combat this.

For example my 4x4 has normal tyre pressures of 26psi all round and the handbook recommends an increase on the rears to 29psi for towing.
I have found 29psi to be totally inadequate for my 100kgs noseweight and rear loading in the boot area so I use 36psi which is fine.
The tyres have a limit of 44psi.

you cannot go saying that! everyone knows one should blindly follow the car manufacturers recommendations! sorry my attempt at humour....Although you have raised an interesting piont with regards towball weights , there are no recommendations for tyre pressure when towing! surely forces work slightly different on parts of the car ie suspension tyres ect that are not accounted for in standard tyre pressure recommendations? now i am actually asking here.....

 

[otherclive]

The important point to remember when towing is to have adequate tyre pressure........too low a pressure on the rear tyres is the enemy.
You do not want over flexing side walls due to increased load which can happen even if you have inflated your rear tyres to the manufacturers towing recommendations.
You can safely increase tyre pressures up to the maximum for the tyre to combat this.

For example my 4x4 has normal tyre pressures of 26psi all round and the handbook recommends an increase on the rears to 29psi for towing.
I have found 29psi to be totally inadequate for my 100kgs noseweight and rear loading in the boot area so I use 36psi which is fine.
The tyres have a limit of 44psi.

I agree that 4x4 can be diffrent. When towing I would increase the Pajero tyres well above the makers recommendation for normal loaded road use. But it does have 15 inch rims with 30 inch diameter tyres, soon to be 33 inch ones. but she's now relegated to moving the van short distances if the main tow car should be elsewhere and its absolutely unavoidable. Which generally is not the case as the van looks as if its about to launch skywards and I feel like a Traveller.

 

[Gafferbill]

Tyresafe......who most agree give extremely good advice on tyres give their recommendations here.
Note their advice on tow car rear tyre pressures when towing.

 

[otherclive]

Tyre safes advice would not have been appropriate for my Sorento where the all round pressure was 37psi which wasn't necessary to increase when towing. It would send our Volvo from 38psi to 45 psi not based on any real understanding of the actual car. I have always gone to the manufacturers max tyre pressure for a fully loaded car. As with two in the car plus dogs plus kit the car is still less loaded than with five people and luggage even wihen the vans noseweight is included. Tyresafe take no account of tyre aspect ratio either.

 

[JonnyG]

Tyresafe......who most agree give extremely good advice on tyres give their recommendations here.
Note their advice on tow car rear tyre pressures when towing.

any advice is decent advice gafferbill, but i dont understand "who most agree give extremely good advice"? whilst i am not knocking their advice,i wouldnt say its was extremely good advice and is generally vague. as otherclive says it has its faults, who is saying they give extremely good advice. As otherclive states it isnt applicable to his vehicles and therefore many others so extremely good? usefull yes..

 

[Gafferbill]

Everybody is entitled to their own view ......... mine happens to mesh exactly with that of Tyresafe which is why I posted the link to their website.