4PSI RULE - Tyre Pressures

Advice on the right rolling stock for your 4WD
Peter Aawen
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Unread post by Peter Aawen » October 2nd, 2009, 2:45 pm

Ambient temp certainly is one of those things that has an impact on your tyre pressures, so what you've experienced hillbilly, is pretty much what you'd expect. Basically, colder ambient temps means that '1 litre' of air will actually occupy less space than it will at higher temps, so while you got away with x litres in Brisvagas, you'll need x=some further south in the colder country, ie you'll need more air inside your tyres to carry the same load, and it's the volume of air that does the work. So you shouldn't be just setting your pressures in Brisbane and expecting the same pressures to work in the quite different environment/ambient temps found in Vic, it's a dynamic/pro-active thing and it needs to be monitored and adjusted as the any of the variables change, ambient temp being just one of them. BUT, by doing the 'hard work' early in the piece and paying attention to what's going on around you and your vehicle as you check and monitor your tyre pressures, it won't take you long to be able to make a good estimate EVERY MORNING as to what impact the ambient temp of the day is likely to have on your 'standard' bitumen road start pressures, or your 'standard' dirt road pressures, or even if you like, your 'standard' wet weather fully loaded bitumen road pressures. So you adjust your cold start pressures accordingly, then use the 4psi rule to fine tune those settings during your day's driving, as the tyres get subjected to all those variables. Make sense??

And certainly your tyre pressure/temp monitoring system can and will help, altho I've previously mentioned the problems associated with external (valve mounted) sensors, and how they are effected by external air temps & conditions more than internal tyre air temps especially when you compare the temps reported to those from internal sensors. Basically, I wouldn't be reading too much into the temps reported by externally mounted sensors, altho any significant changes are certainly worth looking at, even if just for your own peace of mind.

The pressure/temp variances you saw with changes in direction due to the sun heating up one side of the vehicle are also quite common, but you don't necessarily hafta make adjustments immediately due to that sort of 'slight' variation - you need to allow the effect of the road & load on the tyre to take precedence, hence the need to drive for an hour so that those 'slight' variations get swamped by the overriding effect of road surface/load/driving, etc. I often park my vehicle such that one side gets warmed (quite warm) during the early morning, but thru application and monitoring of the 4psi rule I know that the 1-2 psi differences in pressure between the sun warmed side and the shaded side will very quickly be ironed out once I start driving, even just a few minutes at highway speeds. So it comes back to using the 'rule' and getting to know how all those variables effect pressure/temp etc. Actually do it for a while and it'll fairly quickly start to make sense, as you've already noticed.

Trojan, if you are running tyres that are significantly larger than those designed for the vehicle, then you can run into the 'problems' you've mentioned, BUT, since others (I think Grimbo was one of them) mentioned the same effect, I've played around with big tyres on little cars quite a bit, and the 4psi rule can still almost all the time, but there is a point of diminishing returns, ie, your tyres can actually be too big and hold too much air for the weight of your vehicle to actually heat the tyres up to their optimum pressure/temp; so by default you are actually getting less than optimum traction from them, ie not exerting enough pressure on the ground to provide the optimum traction from that tyre. But then if you are wearing tyres that big (or so heavily constructed) under such a lightweight vehicle, then there are probably other reasons that you've decided hold higher priority than the absolute optimum in traction, or have you?? Still, when you run large tyres and get to that point under a light vehicle that even with low tyre pressures they don't change pressure when you drive for an hour or so then you can usually still get better traction by using any one of a variety of methods to either reduce the volume of air carrying the vehicle such that the 4psi rule is satisfied; altho you might not want to add weight to the vehicle, but fitting smaller tyres, running the same OD tyres but on larger diameter rims, or fitting internal bead locks so you can run the tyres at virtually no pressure, et al.

Enjoy ;)
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Unread post by hillbilly123 » October 3rd, 2009, 12:39 pm

Peter, I agree with what u say and just posted to verify thoughts.

Basically the rule applies in your local area and long distance travel will alter things... so you might need to do a recalculation.

What are your thoughts on my last remark about rears heating up with full width mudflap behind them.

I have been told that some vanners have overheated diffs due to air flow being impeded by these things. The air comes in at the front and I would have thought would have found its own way past but my tyre temps suggest other wise.

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Unread post by Peter Aawen » October 3rd, 2009, 2:49 pm

The rule pretty much applies everywhere, even on long distance travel; BUT, just like your vehicle 'glove-box' manual will tell you, or maybe even the tyre placard - long distance travel or fully loaded driving will need slightly higher pressures to normal. So you still use the 4psi rule, but you need to set your 'cold start' pressures in anticipation of the driving you are going to do. So if your normal driving pressures work out at say, 26psi; then if you are preparing for on a longer trip, a fully loaded trip, or today's driving will be on already hot bitumen, then you are likely to need higher start pressures - say 28 or maybe even 30 psi. Just like when you are driving off the bitumen onto a dirt road - you should really be looking at dropping your tyre pressures, or at least slowing down so that you aren't running pressures that are too high for the surface/conditions (remembering that tyre pressure responds to a range of variables, speed & track surfaces being just a couple of them!) Make sense?

Re the flappy things, Lots of modern vehicles use a 'thermo-syphon enhanced cooling system', where the fan itself doesn't actually pull enough air thru the radiator to allow full cooling - the 'syphon' system relies on the air flow from the moving car adding to the total air flow thru the radiator, meaning you can get away with less in the way of power sapping fans. And once the air flow thru the front of the car gets channelled thru the radiator, it then continues past the engine, then down and out under the car, cooling other components as it goes. That down and out bit is aided by the venturi effect created by the high pressure area just in front of the firewall & under the bonnet actually 'forcing' the air down and out, and further enhanced by the air flow over the top of the car and down the back creating a low pressure area at the back of the car, so the air flow is basically sucked out from under - all aerodynamic stuff that can get pretty hi tech; and not surprisingly, even a little change under the car can make a big difference.

And like others, I've noticed that those large flaps across the back behind the wheels can (and do) make a big difference to tyre temps (particularly the rear tyres), and if the ambient/road temp is high enough, they can be detrimental to other things as well. If you ever get a chance to play around with temp monitors on other components, you'll also see that those big flaps play havoc with the temps in the difff & rear axles, and even the temp of the fuel in a tank mounted behind or above the flap.

One vehicle we were running mainly on neat veg oil needed a pre-heater to run properly until we set it up with a full width stone flap behind the rear axle and in front of the fuel tank - saved the need for the pre-heater completely but we couldn't stop the shocks from 'cooking' and fading. When we took the flap off, shocks lasted a helluva lot longer, but we had to re-fit the pre-heater just to keep the engine running. Had us puzzled for a while, until it dawned on us exactly what was going on with the air flow and how much it actually cooled things down under the car.

So if you are already (or are thinking about) running larger than normal 'mud' or 'stone' flaps under the the car, just be aware of the potential for over heating problems, and not just necessarily the engine!! But then again, not everyone works their vehicle hard enough or drives where ambient/road temps make it an issue. F'rinstance, I doubt that anyone who only ever drives in Sthn Vic would even notice the difference, unless they could see it on remote temp sensors (like those in most tyre monitoring systems).

Cheers
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Unread post by Snerk » June 4th, 2010, 11:04 pm

Land Rover recommends using higher pressures in the rear than the front (about 6 psi difference) so how would that be affected if you followed the 4 psi rule?

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Unread post by Peter Aawen » June 4th, 2010, 11:50 pm

Snerk, if the tyre actually needs that much extra air to carry the load you've got up the back, then the 4psi rule will confirm it & still work exactly as always.

What the Landrover recommendation really boils down to is an 'Educated Guess' for what they expect you to be carrying/using the car for; so if you use the 4psi rule to 'Fine Tune' that educated guess to the optimum pressure for what YOU are actually doing & using the car for, it might be a little different or even somewhat different, but it'll still be the optimum for the tyre. Maybe it won't work out at exactly a 6psi increase over what's in the front, but it will still be the 'optimum' pressure for what that tyre is actually doing under the load you are carrying and being driven in the manner you are driving it on the surface you are on.

I've just gone thru this with a fella who was told to run 40 psi in his front tyres & 48 psi in the rear tyres on his vehicle by the people who fitted the tyres. He came on a trip with me and was gobsmacked at where I could drive & yet he couldn't, and that I could show him the log book and receipts showing I got 100,000km life out of each tyre - he never got more than about half that! Checked the sidewalls on his tyres & it said Max cold pressure (when carrying about 2 tonne per tyre) was 38psi! And the vehicle weighed a fair bit less than 8 tonne (2 tonne x 4 tyres on the ground), plus the tyre placard only recommended 30 psi all round unladen. So we dropped his pressures down a fair bit on the bitumen first up - cos they were warm/hot already we went to 34 psi (30 psi cold plus 4psi cos they were warm) and drove for 100 odd km - no pressure variation up front so the 30 psi was a great cold start pressure for the front tyres on the bitumen; but the rears went up by almost another 4psi. That told us that 1. his vehicle was loaded heavier at the rear than the front - true, we could see that, and 2. he needed to start cold with about 32 psi in the rear tyres to cater for his extra load over the unladen recommendation so we added about 1 psi on the spot & noted the need for more air back there for the morning. Next day, he started with those pressure on the bitumen and it all worked out exactly as intended - 30 psi front, 32 psi rear; drove for an hour on the bitumen; checked pressures, 34 psi front, 36 psi rear- spot on!

And off road for the first time in his life he got to let his tyres down below 30 psi!! He was absolutely blown away at what he could drive thru/over with his tyres at 22/24psi (which equates to 18/20 cold start pressures) and the only time he got stopped was his first go on the one big dune that catches almost everyone not prepared for it on the Border track - dropped pressures to 16/18 psi (=12/14 cold) & he idled up and over with no wheelspin at all!! We left the tyres at those lower pressures for the rest of that day, and when we stopped to camp that arvo, they'd gone up 2psi all round indicating that apart from that difficult dune, the pressures we were on now were a touch low, and our earlier 22/24 (18/20 cold) was probly a little high for what we were doing. So when we started from cold the next morning we started on 16/18 cold & an hours driving saw 20/22 psi - spot on the 4psi we were after. Later that day we turned onto some really soft sand and expected to stay in that for the rest of the day, so went back to the 16/18 warm we'd used on the difficult dune the day before, and the tyres remained at that pressure for the rest of the day, again confirming that for what we were doing, the pressures we were running were pretty much optimum (12/24 cold start, 16/18 when warm). And his tyres worked way better for him than he'd ever experienced before, for the first time in his 4WD driving he could understand how people could actually drive anywhere off road without wheelspin!! And I'm pretty sure that he'll now be able to get better than 50,000km life out of a tyre too, cos his tyres won't be over-inflated all the time!

So while the recommended pressures are a good place to start, the 4psi rule allows you to fine tune those recommendations to suit each & every load & surface that you drive on; with that 4psi increase once the tyres are warm (and hours driving) confirming that you are at pretty much the optimum pressure for the best traction, handling, ride, longevity, & performance out of your tyres. Sound like an advert? Sorry, but it still works if you practice & little & use it!! ;)
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Unread post by Peter Aawen » June 5th, 2010, 12:10 am

After quite a few requests about what pressure this 'rule of thumb' will mean for people to run in their particular sized tyres, I need to point out that the sad news (or is it just the big news??) about the 4psi rule is that what works for me won't necessarily work for you! So unless someone else driving say, an 80 series on the same sized tyres, also carries exactly the same load, drives exactly the same roads, at the same times, in the same temps, with the same driving style and braking techniques as you - their optimum pressures will not necessarily be same as yours! Sure, they might be a reasonable indication of what you can expect, but there are enough variables introduced between drivers that even if I drove your car over the same roads at the same speed etc the optimum pressures developed would probly be different!

And if you've been running the 40 odd psi that the tyre bloke put in them when they were fitted & they seem to work OK, while that 40 odd psi pressures may have worked for you in returning a good tyre life or whatever you think is most important, that may well have been at the expense of some other feature, maybe achieving optimum handling, or traction. So if the tyres have made you happy in the way they've worn and behaved, that compromise has obviously worked for you. And only you can make the decision about accepting that 'compromise' to achieve longer tyre life at the expense of (potentially) better traction or handling or whatever. So don't be too put off by the 4psi rule showing a different 'optimum' pressure than what you've used. That optimum is considered to be the best balance between ride, handling, longevity, traction, etc, yada, yada. If you like the ride at 40 psi and the traction still works for you, then go with that, just be aware that you'd probly get better traction (in the dry and the wet) at the figure indicated by the 4psi rule - but that may well be at the expense of longer tyre life. Make sense??

The 4psi rule only gives you a rule of thumb to arrive at the best balance of all those things, and it's not a hard and fast constant or a Law of Nature or Physics, it's a rule of thumb which basically means it gives you a consistent & reasonably accurate way of balancing all those things against one another & taking into account of all the different possibilities in the variables. You don't necessarily have to ONLY vary your tyre pressures to make it work either. If the tyres only go up by 2 psi after an hours worth of driving, you could maybe carry a little more weight, or possibly drive a little faster, and that'd probably let you achieve the 4psi increase without changing the tyre pressures. If they go up by more than 4psi after an hour, toss the kids out of the car, or slow down a bit - there's more than one way to achieve the 4psi increase after an hours driving; just like there's more than one way that each of us may be prepared to compromise the 'optimum' indicated by the rule to meet our own particular needs; and just like the tyre placards, manufacturers recommendations, and even the tyre pressures suggested by the tyre fitter are only recommendations given their particular preference in the compromise from the 'optimum' pressure.

You don't HAVE to use the 4psi rule, but if you do it'll give you what many recognise as the optimum balance between road holding, ride, handling, traction, performance, & tyre longevity - how much you are prepared to compromise to achieve that, to vary from that, or even to ignore it all is entirely up to you! ;)
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Unread post by madmacca » June 5th, 2010, 12:53 am

Just to add from what Peter has said, I use the 4psi rule with my 105 series.

I run 235/85/16 maxxis LT tyres on split rims with tubes and run a cold pressure of 45 to 48 psi depending on my load and what I am towing, I find for me this works in well with the 4psi rule, my tyres also have a max pressure of 80psi. When I have spoken to people about my pressures they are usually gobsmacked that I run "such a high pressure" but one thing I am very aware of is the weight of my vehicle 2800kg unladen.

So I guess as Peter has said, what works with one set-up isn't going to be the best for all, or maybe even for anyone else; you may find that 45 psi is exactly the right pressure for you on bitumen and your tyre wear & handling etc should confirm that. I regularly check my tyres and use a pencil to write on my rim what my gauge has said, I will drop my pressures to 35/40 when doing an extended gravel road trip but will drop my speeds as well. I know a lot of people will say that they cant be bothered checking their tyres that often or that there is no need, but it is my experience that with varied terrain comes the need for varied tyre pressures.
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Unread post by Peter Aawen » June 5th, 2010, 2:01 pm

^^^^ a bit more on that too - the 235/85 is a fairly narrow tyre with a relatively 'small' volume of air inside it, so I wouldn't be at all surprised to see 40 plus pressures in a tyre like that under a heavy vehicle still satisfying the 4psi rule. Same vehicle, same weight, running a 285/75 tyre that has a fair percentage more air volume inside it; I wouldn't expect to see it needing pressures quite so high to satisfy the 4psi rule, unless it was really heavy or there were some other factors requiring/desiring that sort of pressure.

Don't forget that larger volume tyres generally need less air pressure to carry the same weight as a smaller volume tyre - but as mentioned earlier if you want to sacrifice some other feature of the 'optimum' balance of all those affected features, you might use higher pressures to suit your needs/desires. Heavy loads (3 tonnes plus) & long, high speed runs on hot tropical bitumen spring to mind. But it will probly cost somewhat in terms of tyre longevity & outright traction/cornering grip. ;)
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Unread post by Matt1 » June 6th, 2010, 10:00 pm

Bloody hell!! by the time everyone has worked out the 4psi thing, I would of packed up, gone 4wding, come back, cleaned the car and inside sitting in front of a fire place having a few beers:D yes im from melbourne!! very cold!!
If the tyres are not the standard tyres, just pump them up. I run mine at 40psi on hwy, 40 psi in mud, 40 psi over rocky tracks, 40psi in sand, never got myself bogged, tyres still in really good condition and they have nearly done 100000km. yes I run cooper ST.
One tyre has stuffed up due to the CV joint. Nothing to do with pressures. If you want drop pressures when off road, they say to pump them up again when on tar, yes you can do that but you dont need to do it straight away, you can drive on lower pressures but you must drive a lot slower and try not to make any abrupt manovers from the steering. These tyres can run pretty high psi. It says the max on the side wall and of course the recomended settings in the car.

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Unread post by Peter Aawen » June 6th, 2010, 10:19 pm

By doing that Matt, YOU are contributing significantly to the damage that is getting more and more tracks closed to all 4WDers. By running the right pressures in your tyres, and with the right attitude to driving, you'll do a helluva lot less damage to the surface you are driving on, potentially even less than that done by a walker! Besides, when it all comes down to it, you might spend some few minutes in the first few weeks of playing with the 4psi rule, but most people get the hang of it fairly quickly, and then spend a tiny portion of their driving time getting their pressure right - WAAAYYY less time doing that than the time those who don't get their pressures right will be spending on recoveries or fixing vehicle damage!

Your CV joint almost certainly was at least partially due to incorrect tyre pressures/traction, it's amazing how much additional maintenance is caused to vehicles thru them driving on pressures that are too high for the surface, just as it is amazing how much LESS time off work due to injuries & things like 'bad backs' we see in those drivers who always check and run tyre pressures appropriate to the surface they are driving on!

Running the right tyre pressures is just one way you can minimise the damage to your vehicle, your tyres, the track you are on, the environment, and even to your own body; as well as making sure that your vehicle is in a position to maintain traction, stop, and turn etc appropriately when it's on the road too! High tyre pressures, or incorrect tyre pressures on road are a significant contributor to many accidents - a couple of minutes can make your life a lot easier, save a life, and save you a lot of money in less worn tyres, better performance, less accidents, less in the way of ins premiums, and even less in the way of fines, on and off the road. There are more and more tracks where the damage caused by people running high tyre pressures has been so bad that now if you are caught on them with pressures above those recommended for the track, you can be charged and fined for it - that's only where the track managers have taken the approach that it's only a small minority that can't/won't do the right thing and drop their pressures appropriately, and that minority shouldn't get the tracks closed for the rest of us. Sadly, those track managers are also in a minority, and most tracks that are damaged thru the selfishness of those vandals who won't lower their pressures appropriately simply get closed to everyone!

By putting a little effort into getting your tyre pressures right, you will be helping to keep tracks open longer, and you'll be helping yourself become a better driver.
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Unread post by Matt1 » June 6th, 2010, 10:50 pm

I do agree in what your saying Pete but I believe you can drive tracks without lowering your tyre pressure and not damage the tracks as long as there is no wheel slippage, deliberatly trying to spin the wheels or driving off the designated track. I believe that even lowering the tyre pressure can be just harmful as not lowering the pressure. If you think about it, Yes you get more grip thus less wheel slippage but when you do, you will be digging out more dirt than a higher pressure tyre. I do assure you I do tread lightly though! Please correct me if im wrong!!

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Unread post by Agent 13 » June 6th, 2010, 11:07 pm

Matt1, I can never understand not lowering tyre pressures offroad, be it by the 4psi rule (which I do find a tad over scientific especially when driving all day over variable terrain...) or my standard of drop em to 24 or so the minute you leave the main roads. Hard tyres offroad = hard ride (eg on corrugations or rocky tracks), less stability, greater risk of puncture and definately greater wheel slippage on steep rocky tracks or in the mud. I cannot see an arguement for driving tracks at 40psi. Yet plenty of outback drivers seem to do it. My limited experience on outback roads, I cannot understand it at all... bounce all over the place on 40psi, or glide over everything at 24/26 whatever.
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Unread post by bozo21 » June 7th, 2010, 1:22 am

I was told by a tyre expert that the 4 psi rule only applies to passenger construction tyres, not LT tyres. LT tyres need a change of 6psi, as they run hotter and also need more pressure then passenger construction.

So really for 4wd tyres it is the 6 PSI RULE. Although i still find it strange, becuase lets just say i have 35 psi cold, and go for a drive and it only reaches 38psi because i over inflated them. I drop them down to 32psi and go for drive and they reach 38psi also. Which under the 6psi Rule is perfect ok. BUT, why isnt it perfect to run them at 35psi because they were at 38psi when warm also. Stupid really, your tyres should be at the optimum pressure that they have grip, produce less drag and are sit perfectly level with surface.

Its just all a little overboard i reckon, safety shouldnt be compromised for longevity of tyres. If there hard and vehicle handles well, then thats the correct pressure. Some people look to far into things.

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Unread post by Matt1 » June 7th, 2010, 1:26 pm

Agent 13, I guess its harder for me to tell as I have suspension seats!:D Dont get me wrong I am all for lowering tyre pressure but what Im saying is that the 4psi is a little over the top and just to pump the required amount of air into the tyres!! and read the guage!! The only thing Ive been told is that you dont pump the tyres up when they are hot. Only do it when they are cold or reasonably cold. As for 4wding I'm just stating a fact is that you can drive sensibly and not damage tracks with higher pressures. I admit I havnt been on real difficult tracks so there is probably a difference there, but with the tracks I have been on there hasnt been a requirement to lower pressures and I havnt damaged the tracks. but thats my opinion anyway!! What I stated in my first comment ( about running tyres at 40 psi in different conditions) is true. but not all of those conditions were in a State park and / or designated tracks. Some of those conditions were on property. (yes family property!) wasnt illegal:D

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Unread post by Peter Aawen » June 7th, 2010, 5:55 pm

The 4psi rule (or if you like, 6psi rule - Coopers & others are advocating that for their LT tyres) is just a 'rule of thumb' to allow any operator to consistently determine that the 'optimum' pressure AND TEMPERATURE of the tyre has been reached for it to operate at its best. And that's why there's a difference between putting 35 psi into your tyres cold & ending up at 38 psi, or putting 32 psi into your tyres cold and ending up at 38psi. In both cases the final result was 38 psi, but the internal temp of the tyre & the air in that tyre will be somewhat different. For starters, that means that the 35 psi start tyre will have less than optimum traction cos the rubber/compound is colder, and it'll probly also have less ability to mould over the terrain (therefore less puncture resistance) and a few other things too.

All up, the 4 (or 6) psi rule is simply a quick and easy way for ANY driver to consistently set their pressures in order to get the best from their tyres - and really, the difference between the two isn't all that critical, just so long as whatever rule you prefer is consistently applied. And despite it looking complex to start with, once you've got the hang of it, it really only takes seconds to sort, and a very tiny percentage of your overall driving time, and yet it provides MASSIVE benefits over the life of the tyre and during each drive. Every time I take a new group out on a trip, there is someone who scoffs at it, and yet the further we go the less scoffing there is; cos having tyre pressures that are appropriate to the surface you are on and the conditions you are driving in is such a critical thing to your vehicle's capability & performance off road, let alone the benefits to be had in tyre life.

And I've said it quite a few times before, once you have an idea of what you are doing, you don't hafta get out and adjust tyre pressures at every surface change - you can if you wish; but it's easy enough to vary your speed, the way you are driving, or one of the other factors that contribute to tyre pressure variations just enough to account for the different surface. You'd be surprised how quickly you can learn what varies pressures and by how much, and you'd be surprised at how little effort it is to make the changes necessary to keep your tyres running at their 'optimum' pressure. So Matt, I'm basically agreeing with you about not having to adjust pressures for every surface change, but you DO need to adjust one of the factors that contributes to tyre pressure/temp, preferably one that you have some control over; and the easiest one of those to vary is usually speed. But if your tyres are way higher than the optimum for the surface, the drop in speed required can be significant and really not a viable option - then you can simply drop your tyre pressures instead! ;)

Sure, you can do a lot of driving without changing your tyre pressures or varying any of those other factors, and lots of people do that, but they (knowingly or otherwise) will be getting less than the best from their tyres; doing less than the best for their vehicle & their bodies; and even with appropriate & skilled driving, they will be doing less than the best to the surface they are driving on. That same appropriate and skilled driving on 'optimum' tyre pressures will do far less damage than the same driving done on tyres with pressures too high. And we all pay the penalty for that, in track conditions & closures amongst other things.

So if you can't be bothered using appropriate tyre pressures for yourself, despite the potential savings in tyre wear, vehicle wear, occupant comfort, vehicle handling & performance, etc, etc; then at the very least you should be doing it so you don't damage the track more than necessary and ALL of us don't suffer from more track degradation and/or closures. Doing anything less is extremely selfish and self-centred, isn't it?? ;)
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