Car tyre pressures

[Royston]

Just been pumping up my Mondeo car tyres ready for our first towing with this car,.
Just given it a thought really, do you maintain the increased tyre pressure whilst you are on site even when driving solo, or do you reduce the tyre pressures to normal running and increase them prior to leaving site.
The tyre pressures on the Mondeo have 1psi increase on the fronts and a 9 psi increase on the rears.for towing purposes. On my previous car i used to leave the increased tyre pressures on all journeys when i was away, and only reducing to normal running pressure upon return.
Royston

 

[NigelHutson]

It depends how long I'm away for, and whether we're going to use the car much. If it's just a weekend, where we might just *** out, then I'd leave them as they are, but if we're on a site for a couple of weeks, then I'd return them to their "normal" pressures.

 

[TonyH1]

Hi Royston,
That's quite a big increase on the rears so I think I'd be adjusting them back upon arrival although it may depend on how loaded the car is for the rest of the holiday. Does your manual recomend increased pressures just for being "4 up" rather than "2 up"?

Regards

 

[Royston]

Hy Tony H ,
My Ford manual states the following Tyres 235/45/R17 Normal Load Front 35 Rear 32, Full Load Front 36 Rear 41.PSI Taking in the facts that with the weight of the van it must come under the Full Load option.
I have just recently towed using the heavy Load figures 90 miles in normal sunshine , return trip 90 miles in heavy rain, speeds up to 60 mph both the car and the caravan towing was spot on no sign of any vibration from the van.
I now know what a excellent all round car the Mondeo is for both solo and Towing, must admit surprised at the lack of co-respondense from other Mondeo owners , with regards to various inputs on the Forum.
Royston

 

[ProfJohnL]

Hy Tony H ,
My Ford manual states the following Tyres 235/45/R17 Normal Load Front 35 Rear 32, Full Load Front 36 Rear 41.PSI Taking in the facts that with the weight of the van it must come under the Full Load option.

Hello Royston,
You should always follow the car manufacturers recommendations regards tyre pressures and loads, but your last sentence above is not necessarily true.
Consider the fact that the majority of caravan nose weights are around the 75Kg level, which is the equivelent of an adult standing on the tow ball. So if the car was otherwise empty but towing, and allowing for the extra leverage of the weight on the towball overhang, the load on the car would be less than having the boot full of peat from the garden centre or two adults in the back seat.

 

[otherclive]

Prof
peat........surely you mean compost from environmentally sound sources? Perhaps sand might be a better example of boot cargo )

 

[Royston]

Thanks for your comments Prof,
I do load the rear seats with items also the rear floor area probably equivalent to two teenagers, plus the Hatchback boot is filled up with additional items, The fact that the car towed the caravan with the tyre pressures quoted with ease passing heavy lorries without the slightest twich must mean that I am in the right Ball Park.

Royston

 

[otherclive]

Whilst not a Mondeo my XC70 has a full load option of 38psi ( up to 100mph cruise speed!) all round and I tow at this pressure. But the car can also be driven solo at 38 psi as Volvo call this an Eco setting. But the cars normal tyre pressure for up to 3 adults and kit is 33psi. So for short trips I keep the tyres at their towing pressure but if we are away for longer I drop them to 33psi as the solo ride at 38psi can be a bit fidgety and I would worry about uneven tyre wear. But it seems to me Volvo have effectively said for solo you can use anything between 33 and 38 psi. Bizarre as in Eco setting the ride is a bit fidgety and fuel economy doesn't change.

 

[TonyH1]

Hi Royston,

Your further comments really confirm for me that you're using the correct pressures while towing but in answer to the original question that you ask, and with the presumption that you're usually only "2 up" and would have unloaded most of the other items then I would be reducing the tyre pressures once arrived on site.

Regards

 

[emmitdb]

Greetings,

This thread has just got reaching for the slide rule (God!!! Am I THAT old???)

I've just been calculating the load on my vehicle (AudiA6 Avant) with the the 'van attached and with an 85Kgs downforce on the ball.
It works out at a load of 128Kgs on the back axle with a corresponding reduction of load of 43Kgs on the front axle.
Not quite two adults (not in our house anyway) but not far off.

 

[brian_c]

Hi Royston. I tow with a 2 litre diesel Focus. I always increase my pressures to the full load setting and return the pressure to normal once at site. Then back again for the return journey. I have not looked any further than the comment in the hand book re-normal two up loading and full loading. Ie:- four persons plus luggage. So I don't have any real technical back up to my reasoning other than that. However when at home and my tyres set to the lower pressure if I occasionally give a lift to other people therefore loading my car I never think to increase the pressures then. Double standards I suppose.

I have a Ring battery operated inflator and my only calibration standard is that when I first bought it I compared it's setting to my old fashioned pressure gauge, Also a couple of times I have checked at the local petrol station using their alleged calibrated gauge and the readings compare quite well but still that is not really scientific. I feel that I am at least doing what is the right thing.

Cheers for now Brian

 

[Royston]

Thanks for all your comments, reason I asked the question is, I have never ever seen anybody pumping up their tyres prior to leaving campsites, they may of course pump their tyres up on garage forecourts after filling up with fuel . I must admit I never use that facility always use my footpump and calibrated hand guage prior to leaving sites.
Royston

 

[Surfer]

We don't use forecourts any more as we carry a compressor with us at all times. I have to admit we hardly ever check tyre pressures.

 

[otherclive]

Thanks for all your comments, reason I asked the question is, I have never ever seen anybody pumping up their tyres prior to leaving campsites, they may of course pump their tyres up on garage forecourts after filling up with fuel . I must admit I never use that facility always use my footpump and calibrated hand guage prior to leaving sites.
Royston

whats your calibrated hand gauge as I have two 'pencil' type gauges and two quite expensive digital ones and none agree. There is a 4psi difference on the digital ones but only 1 psi between the pencil ones. But which one is most accurate? I tend to rely on the pencil PCL types as that sort held me in good stead with motorbikes and I don't get adverse tyre wear. Thing is both digital ones claim to be accurate to 1% which is 0.3 psi in 30psi.

 

[RogerL]

In all the tests I've ever seen, the pencil gauges are the most accurate - digital ones can/do vary.
My electric pump has a digital gauge - but I over-pressure the tyres slightly and then use a pencil gauge to set the correct pressure.

 

[Surfer]

In all the tests I've ever seen, the pencil gauges are the most accurate - digital ones can/do vary.
My electric pump has a digital gauge - but I over-pressure the tyres slightly and then use a pencil gauge to set the correct pressure.

Although I have the Michelin digital tyre pressure guage, I also check the tyre pressure monitor inside the vehicle to make sure that all the pressures are the same.

 

[otherclive]

But surely your Michelin gauge is sufficiently consistent to be confident that all tyres are at the same pressure without needing TPMS to confirm it. My grift is the apparent inaccuracy or inconsistency between digital gauges supposedly made to the same level of accuracy. Even 3 percent accuracy should only give +/- 1 psi at 30 psi, which would be nice to achieve as at its worst case the difference would only be 2 psi.

 

[Royston]

Guess the guage I use is what you are calling a pensil guage,It relies on the air pressure released throught the car valve to push the the inner part to register a reading on the external scale. Have found it to be more acuarate than the digitial ones, Present car has alloy wheels ,,the rubber air valves do not protrude very far out of the wheel trim, so getting the the guage to sit properly on the air valve is quite tricky, I got fed up with the digitial guages showing zero readings along with air escaping throught the connection.
Royston

 

[otherclive]

Guess the guage I use is what you are calling a pensil guage,It relies on the air pressure released throught the car valve to push the the inner part to register a reading on the external scale. Have found it to be more acuarate than the digitial ones, Present car has alloy wheels ,,the rubber air valves do not protrude very far out of the wheel trim, so getting the the guage to sit properly on the air valve is quite tricky, I got fed up with the digitial guages showing zero readings along with air escaping throught the connection.
Royston

Likewise, sometimes the old ways are the best.

 

[MichaelE]

I have tended to run my car tyres inbetween solo and fully loaded and left them at that, if you leave then on them high side the tyres wear in the middle, I couldnt keep inflating and deflating all the time.

 

[JonnyG]

time to state the obvious. all recommended Tyre pressures are cold reading, secondly the car handbook is recommended pressures and are not legally bounding nor should one go assuming that the car manufacturer knows best given that the same Tyre on a different make could have smaller or a wider psi window for its car. after all some car manufacturers recommend numerous readings for many differing examples compared to others...even when weight distribution is similar...the Tyre sidewall gives you the max Tyre pressure one can use legally. between that and any recommendations the car manufacturer makes is a wide scope of settings that are more to do with how you, the driver feels is most suited....also remember those so called recommendations are based on what the car manufacturer fits as standard ie the make of Tyre and another make of Tyre will not necessarily feel or work the same at the same psi as the standard fit Tyre make...if all things are equal even on the max psi setting tyres should not wear much more in the middle than nearer the sides.
something would be wrong tracking camber worm suspension as if it is wearing drasticly more so in the middle your contact area is not as big as it should be, or you gauges are reading far too low [unlickly] and your tyres are over inflated....above the max psi

 

[WoodlandsCamper]

...the Tyre sidewall gives you the max Tyre pressure one can use legally. between that and any recommendations the car manufacturer makes is a wide scope of settings that are more to do with how you, the driver feels is most suited....

You haven't mentioned the other bit of information printed on the side wall that goes with max pressure, viz @ a given load on the tyre. If the load is reduced then the inflation pressure should be reduced. That is why manufacture's handbooks state that you should increase the tyre pressure when going from 2-up to a full load.

 

[JonnyG]

...the Tyre sidewall gives you the max Tyre pressure one can use legally. between that and any recommendations the car manufacturer makes is a wide scope of settings that are more to do with how you, the driver feels is most suited....

You haven't mentioned the other bit of information printed on the side wall that goes with max pressure, viz @ a given load on the tyre. If the load is reduced then the inflation pressure should be reduced. That is why manufacture's handbooks state that you should increase the tyre pressure when going from 2-up to a full load.

you'd think so but alas that is totally incorrect load rating is the max weight a tyre should carry there is no proper colloralation given by anybody between the load rating of a tyre and any psi recomendation by car manufacurers .for instance load index 91 equal 615 kg and is standard fit on my mondeo which mean the back axle max can carry 1230kg max the tyres have a max psi of 50..... if as you say there is a colleralation then the handbook should state that when fully loaded my tyre's are pumped upto 50psi...and you wont find that in any ford mondeo book...infact go check any vehicle tyres that have the correct tyre fitted and you'll never find the max recommended by a car manufacurer matching the Max psi of a tyre....infact my rear have a max of 50 psi and the fronts[different make] but same load index have a max of 46psi!!!!!
I'd say recommended tyre pressure are a good guide and starting piont but no more than say the 85% rule is for towing..

 

[WoodlandsCamper]

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.

 

[ProfJohnL]

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.