Tyre pressures: Cold vs hot

Submitted: Wednesday, Feb 20, 2008 at 21:48
ThreadID: 54766 Views:3068 Replies:12 FollowUps:12
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Hi All,

I'm currently about 2 months into our 6 month trip and I've been playing lots with my tyre pressures, with some rough tracks and beach driving. I'm curious as to how other people set their tyre pressures as I use to just set them cold and forget about them but obviously pressures rise when you start travelling and heat biulds up in the tyre.

I thought there was a rule that your pressures should only rise a certain amount from cold and this indicated if you were setting the right pressures when the tyre was cold. Eg. if this figure was 5 psi, setting at 30psi cold then when hot they should only rise to 35psi; under or over this and you haven't got the right pressure. Anyone heard of this?


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Reply By: Footloose - Wednesday, Feb 20, 2008 at 21:53

Wednesday, Feb 20, 2008 at 21:53
Yes, but I was told 4psi was the rule....pretty close :)
AnswerID: 288528

Follow Up By: Richard Kovac - Thursday, Feb 21, 2008 at 00:35

Thursday, Feb 21, 2008 at 00:35
5 psi is for tube tyres 1 psi for the tube


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Reply By: Member - Norm C (QLD) - Wednesday, Feb 20, 2008 at 21:55

Wednesday, Feb 20, 2008 at 21:55
Hi Mark,
I know it as the 4PSI rule. If you set your tyre pressure when cold, then go for an extended run, the pressure should increase by about 4PSI. If it increases more, the cold pressure was too low. If it increases less, the cold pressure was too high.

So yoou have the right concept, but AFAIK, it is 4PSI.

Norm C
AnswerID: 288529

Reply By: Member - Mark H (VIC) - Wednesday, Feb 20, 2008 at 22:01

Wednesday, Feb 20, 2008 at 22:01
Thanks Norm & Footloose. I had a guess at 5, glad I know the right figure now.


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Follow Up By: Bware (Tweed Valley) - Wednesday, Feb 20, 2008 at 22:47

Wednesday, Feb 20, 2008 at 22:47
I'm in agreement with Norm and footloose on that figure, but were you asking in regards to correct pressures for 'rough tracks and beach driving'? If so, I don't think the same rule would apply.

If you were beach driving in semi-soft sand at 32psi, after an hour your reading may be around 36psi (correct reading). Just because the sand is soft (or the road is corrugated), doesn't mean your reading will change a great deal. The tyres will get to operating temps and give a particular read. That doesn't mean it is the 'correct' temp for off-road conditions. In soft sand you may run 18 - 20 psi so you don't get bogged; I'm sure a hot reading would indicate that you were running the incorrect pressure.
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Follow Up By: Member - Mark H (VIC) - Friday, Feb 22, 2008 at 00:17

Friday, Feb 22, 2008 at 00:17
Nah I was just wondering about the 'rule', no way would I drive on sand at 30odd psi unless it was like concrete. I just meant that in the last 8 weeks I seem to be blowing up the tyres every week, because I have dropped the pressures for rough rocky tracks or for some sand driving. Thanks all the same.

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Reply By: Member - Phil G (SA) - Wednesday, Feb 20, 2008 at 22:48

Wednesday, Feb 20, 2008 at 22:48
Its called the Pirelli rule. Its a good guide, but don't take it too literally, otherwise you can end up with ridiculously low pressures on the highway, and too high pressures in the rough.
AnswerID: 288544

Reply By: splits - Wednesday, Feb 20, 2008 at 22:58

Wednesday, Feb 20, 2008 at 22:58

I fitted a new set of tyres late last year and decided to ring the tyre manufacturer and discuss pressures with them. These tyres were a type I had never used before. I was told to use the pressures recommended in the car owner's handbook and never exceed them by more than 4 psi. I mentioned the 4 psi rule and he said there are so many variables in it that every tyre testing program that he has been involved in showed that it was not reliable.

The most importand thing you have to worry about with tyres is how they are wearing. Underinflation will wear the outer edges of the thread faster than the centre. Overinflation will wear the centre faster. If the pressure is right then the wear will be even and the ride and handling will also be right.

For as long as I can remember I have always marked tyres so I could watch how they were wearing. I simply get a small nail, heat the head red hot and touch it lightly about five or six times across the surface of the thread. This will leave tiny dimples about half a millimetre deep. By watching these dimples you can easily see if the tyre is not weaing evenly long before you can detect uneven wear by any other method.

I am now up to 4,000 ks on these tyres with factory pressures and the wear is perfectly even. The ride and handling is also excellent. This distance included 900 ks in the Vic. High Country in dry conditions. I have noticed the rear wear faster than the front so I have started rotating them already to try and keep them even.

I suggest you mark your tyres and give their manufacturer a ring. It only takes a phone call and who knows what you will find out.

AnswerID: 288547

Follow Up By: Member -Signman - Thursday, Feb 21, 2008 at 11:51

Thursday, Feb 21, 2008 at 11:51
Why don't you just use a tyre depth gauge?? A lot of tyre outlets give them away as a promotional item...
Not only will it indicate even/uneven wear, but actual wear rate also..

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Follow Up By: splits - Thursday, Feb 21, 2008 at 20:00

Thursday, Feb 21, 2008 at 20:00
I tried those years ago. I found I had to drive too far before I could see any difference. The little marks I have been usiing will show a difference almost immediately. They are no trouble to put on and are very easy to see, much easier than carefully placing a gauge in a number of places across the thread.

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Reply By: Gronk - Wednesday, Feb 20, 2008 at 23:59

Wednesday, Feb 20, 2008 at 23:59
The 4 psi is a good starting point, and used in conjunction with a bit of experience and common sense, will see your tyres performing well.....

Say your tyre placard said 30psi....if you were going on a trip....then you would probably add 4psi for high speed highway driving (as it says in the handbook !! )
Then you would maybe add another 4psi to compensate for the crap you have in the back and maybe the CT you were also towing ???? Too late after your tyre blows out to work out your pressures were too low !!!!

Now if you were going to hit the dirt.....and still travel at say 80 to 100 k/hour, then leave the pressures the same, but a different story if you intend to go a lot slower !!!!

Now sand ????? temps will go up a fair bit in sand but you have no choice ( if you don't want to get bogged )
AnswerID: 288562

Follow Up By: splits - Thursday, Feb 21, 2008 at 01:33

Thursday, Feb 21, 2008 at 01:33
The owner's handbook for my car does not say to increase the pressures for high speed driving. The books in some of the other cars that I have owned did say to increase them.

It lists the same pressure for the front and rear when the car is empty. The front remains the same when fully loaded but there is a 9 psi variation listed for the rear when loads are added. The weight on mine is a fraction over half the maximun carrying capacity so i increased the rear by 5.

It also lists another stock size tyre but with a different load rating. The front pressures are the same for this tyre but the rear is higher.

Obviously sand and mud driving will require some variation in pressures but these are usually short term or low speed activities and don't seem to cause major temperature problems.

As the tyre company said, there are a lot of variables and the situation could easily vary from car to car and tyre to tyre. In my case I have a tyre company telling me their tyre will perform perfectly with the car manufacturer's recommendations and everything seems to be working so far.


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Reply By: Member - Barnesy - Thursday, Feb 21, 2008 at 03:34

Thursday, Feb 21, 2008 at 03:34
I'm struggling to understand the point of this "4psi rule". It seems like a lot of guess work and mucking around.

Why not simply check and adjust the tyre pressures whilst hot?

AnswerID: 288570

Follow Up By: Gronk - Thursday, Feb 21, 2008 at 08:58

Thursday, Feb 21, 2008 at 08:58
Check and adjust the pressures to what ??? You need to know what pressures you started with ???

I have a terracan and run 38 all the time and don't usually adjust pressures up to suit high speed or heavy load driving as I'm already running higher than normal pressures.....

For the lazy people ( like me ) better to have too high a pressure than too low !!!!
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Follow Up By: Member - Barnesy - Thursday, Feb 21, 2008 at 15:12

Thursday, Feb 21, 2008 at 15:12
>>>Check and adjust the pressures to what ???

I agree with the post below. The pressures need to be adjusted as you change road conditions and speeds. If you simply put air in the tyres cold and leave them all day (guessing the 4psi rule) then who knows what pressures they actually are?

You admitted you're lazy, get out and check the pressures after a bit of driving!
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Follow Up By: Gronk - Friday, Feb 22, 2008 at 08:26

Friday, Feb 22, 2008 at 08:26
I think a few of you have missed the point........after playing around with pressures and applying the 4psi rule, I know what cold pressures I can put in to cover most loads and outside temps, so when I'm out in the outback I don't need to worry about playing with pressures !!!!!!

When you say the pressures NEED to be adjusted for conditions and speed........ONLY if you want to go to the trouble of playing with them ???

A tyre won't blow out if your pressures are 6psi too high, but try them at 6psi too low ??

I'd rather keep on the slightly high side ( rather than change a tyre in the outback )

But having said all that....if I was going to do an all day run at 60k/hour along creekbeds etc then I would probably reduce the pressures !!!!!
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Reply By: Anthony (Vic) - Thursday, Feb 21, 2008 at 08:44

Thursday, Feb 21, 2008 at 08:44
G’day MarkH

I threw the 5psi approach out the window for our last trip with the weather well over 40C each day. I did change the tyre pressure to suit the conditions. The tracks we travelled varied but mostly had corrugations, ruts and stones with the odd bit of sand encountered.

We went as low as 23psi (front cold) and 27psi (rear cold). As the days progressed these pressures would climb least 10 -15 psi. I would then stop during the day and reduce the pressure in the tyres to improve ride comfort and reduce tyre bounce and sideways hopping on corners. We didn’t travel to fast for the track and varied between 40 and 80kph depending of the conditions.

I don’t get too concerned about lower tyre pressure and side wall damage. IMHO I think side wall damage is just as likely to occur at all pressures and varies with the different types of stones on tracks across Australia. I believe that keeping a keen eye out for the large and/or sharp looking stones and being able to dodge these buggers is one of the answers to extending your tyre life.

cheers Anthony
AnswerID: 288581

Follow Up By: Gronk - Thursday, Feb 21, 2008 at 09:07

Thursday, Feb 21, 2008 at 09:07
Tyre pressures would climb 10 to 15 psi ?????

Out in the boondocks I would take a hard ride over a possible tyre blowout !!!!!

If you reduce pressures cause of track conditions ( and you reduce speed as well ) all and good, but where people come undone is when the track returns to normal the speed goes back up and "bang", I've blown a tyre on a nice flat piece of road ???? Must be something wrong with those "bleep" brand of tyre ????

If you do 80 to 100k/hour on tar, then you need the same pressures on dirt if going the same speed !!!!!
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Follow Up By: Anthony (Vic) - Thursday, Feb 21, 2008 at 09:22

Thursday, Feb 21, 2008 at 09:22
H'day Gronk,
Yep ... 10 -15psi. That is an increase on cold psi and it was darn hot, at 45C on some of the days travelling.
And I agree, the pressures need to go back up to suit when you get back on the tar. I didn't mind stopping and taking the time to do just that ... I was on holidays after all and in no particular rush to get anywhere.
Cheers A

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Reply By: Member -Signman - Thursday, Feb 21, 2008 at 11:46

Thursday, Feb 21, 2008 at 11:46
I wouldn't be too perdantic about 4 or 5 psi...Most tyre gauges are a few psi inaccurate anyway..
4psi on your gauge may be 6psi on mine & 5 psi on someone elses..
AnswerID: 288600

Follow Up By: Gronk - Thursday, Feb 21, 2008 at 13:58

Thursday, Feb 21, 2008 at 13:58
I would agree........4,5 or 6psi......but the general rule would still apply.....

My placard says 30psi.....but I run 36 to 38 all the time so I don't worry about adjusting pressures before going on a trip....and I don't let them down for dirt roads either ( sand is another matter )

I found on corrugated roads ( tracks ) that no matter what pressure I was running, it still came down to finding that particular speed that gave the best ride !!!
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Reply By: PradOz - Thursday, Feb 21, 2008 at 22:17

Thursday, Feb 21, 2008 at 22:17
the 4 psi rule explained

I think you should check out the link which will explain the 4 psi rule clearly (plus some more info too)

It states :
It is impossible to list the correct pressures for every caravan, due to variation in size, load, etc. This easy check will help you find the best pressure for your caravan tyres.

This is the info i have seen on many professional web sites and motoring companies. I have used it and it works well. good luck

AnswerID: 288735

Reply By: Member - Mark H (VIC) - Friday, Feb 22, 2008 at 00:35

Friday, Feb 22, 2008 at 00:35
Thanks everyone for your input. I'm getting lots of chances to experiment with this so I'll do some testing and report back on here in the future with how my set-up performed.


AnswerID: 288752

Reply By: Member - Mark E (VIC) - Friday, Feb 22, 2008 at 09:25

Friday, Feb 22, 2008 at 09:25
I'm sure to get a few gasps here, but I tried running the 4psi rule with my split rims/tubes/Road Grippers last year on a trip across to WA from VIC, with fully loaded vehicle with fuel, water, camping gear, spares etc. To be able to limit pressure increase to 4psi at about 90 to 100kph on the bitumen, my pressures needed to be up around 70psi for the rears and a little less on the fronts. Funnily enough, with pressures less than this, the tyres 'looked' bagged out too much for the bitumen, but with 70psi, looked about right.

I did however accept a higher rise in pressures on dirt roads as I didn't want to run those pressures for ride reasons. Pressures for the whole trip were greater than I have ever used and funnily enough I didn't get one puncture in over 12k km. Tyre wear has been quite good since with a further 20k km on bitumen. These 'cheapo' tyres (bought as 'roaded' from the wreckers) look like they're going to last for about 60-65k km which is more than I have ever had from these tyres in the past. As a result, I feel that this particular combination of load, tyre, rim, car seems to suit higher pressures on 'hard' roads. It may not suit everyone, but I'm continuing with it for the moment. Around the city, I run about 50psi unloaded, and given the nice suspension, I don't feel the ride is overly harsh at all.


AnswerID: 288773

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