On Road Tyre Pressure

Post by **greg** » Fri Sep 12, 2003 5:04 pm

Hi All,

I'm just wondering if anyone has a whiz bang formula for working out correct tyre pressure for on-road driving.

Basically we've just reduced the weight of my car by a heap (over 100kgs I would suspect), and I think it's time to work out what the "correct" pressure to be running my tyres at is going to be.

Here are the stats incase anyone has some ideas:

Tyres - 34x9.5x15 Super Swampers - they are load rated for 2370lbs - which at approximately 2.2lb per kilo is about 1000kg's each right? They also have a max inflation of 45psi...

see here:
http://www.intercotire.com/html/special_service.htm

I would estimate that the weight of my car now sits at approximately 800 or 900kgs - but i'm yet to confirm that.

So, is the calculation something like?:

new pressure = ((Car Weight / 4 Tyres) / max load) x max pressure

new pressure = ((900 / 4) / 1000) x 45

new pressure = (225 / 1000) x 45

new pressure = 0.25 x 45 = 11.25psi

Any ideas?

Thanks,
Greg

I recall someone having a rule of thumb about checking psi after driving car for an hour to test if it has gone up or down 4psi or not? Any ideas?

Post by **Singo17** » Fri Sep 12, 2003 5:29 pm

I don't know about ya fangled calc but the rule of thumb was supposed to be a 4psi rise from cold to hot running (around an hour). I don't really follow this rule but mine does comply. GU at about 3-3.2 ton loaded MTR's 285/75's running at 45 psi cold and 48-9 hot. I run them this inflated as they are an LT. Inflate up to 55 psi. I find heaps better handling at this pressure on road with little discomfort. On dirt it is to high drop em to 35 psi and if rough dirt a little lower to reduce staking at speed.

This is just opinion only of course.

Swampers have that raised centre tread so I would think you would not have to inflate nearly as high considering the weight. But more air in would help handling and rolling resistance on road. Why not just experiment with em say start a 15psi if it feels sqirmy on the road more air (remembering you need enough air in them not to overheat the carcas ) .

Post by **hazard** » Fri Sep 12, 2003 6:12 pm

The rating on the tyre side wall just means that if the tyre is at is maximum load it must have at least the stated tyre pressure.

The on road tyre pressure has to be fairly high to minimise side wall flex = heat = the number 1 cause of tyre failure.

Pressures that are too low are dangerous and chew out the edge of the tyre. Start at 35psi and look at the tread to see how the tyre wears or even better use a tyre pyro to get even temperatures across the tread. Too much pressure = high temp and wear in the centre of the tread.

Post by **Singo17** » Fri Sep 12, 2003 6:30 pm

Does it matter that it isn't a radial we are talking about.

I thought the rating on the side was the MAX load you could carry on the tyre at that pressure. Otherwise we should really weigh our trucks and find out what pressure we must be run.

Post by **big red** » Fri Sep 12, 2003 7:56 pm

i just use the "4psi rule"
check tyre pressure when cold then drive up the highway for an hour and check pressures again.
if the tyre has gone up more than 4 psi then the start pressure was too low and tyre has overheated causing pressure rise.
havent had the swampers long and probably wont drive on them for an hour up the highway but it works for the BFG MT's.
shane

Post by **redzook** » Fri Sep 12, 2003 8:09 pm

hazard wrote: Start at 35psi

the tyre placard(SP?) in my sierra
only says 20 psi for street drivin
so with a lighter then standard id say bout 15 to 18 psi

Post by **moose** » Fri Sep 12, 2003 10:45 pm

I,m with redzook
I run my 33,s (on a zook) at 18 psi , on road !!!!

anymore , it rides harsh & NO grip in the wet (as swampers go !!! :lol:

)

anything below 15 psi & it,s a bit sloppy !!!!

Website · Post by **Maggot4x4** » Fri Sep 12, 2003 11:04 pm

My 37 MTR's get run at 25psi on the road and the swampers at 20psi.

3+ tonne 80 series

Post by **mike** » Sat Sep 13, 2003 5:51 pm

On my spook I run My Yoko 31 10.5 R15s @ 18psi
and my 31 12.5 R15 Bogger TSLs @ 7-8 psi anything more and I don't get a full contact patch

Post by **dumbdunce** » Sat Sep 13, 2003 8:17 pm

it's probably bad, but I deliberately overinflate my tyres on road by about 6 psi, for a couple of reasons - off road I wear the shoulder lugs/outside tread real fast, due to underinflation and rocks, so it evens up the wear - also helps the tragic 2.4 pull the truck along, improves fuel economy, and reduces steering effort. the wear looks fairly even across the tyre so it's not all bad. maybe worse grip in the wet but it's not the sort of truck you can throw into corners etc at the best of times.

"ideal" (by the 4psi method) pressure method for me is around 33 - 35 psi so I shoot for 40 most of the time.

cheers

Brian

Post by **taziiy** » Sat Sep 13, 2003 8:51 pm

i fit tyres part time on sat when ever we put new tyres on 4x4's we inflate them to 40psi thats just wat i was told to do.

Post by **mike** » Sat Sep 13, 2003 11:57 pm

taziiy wrote:i fit tyres part time on sat when ever we put new tyres on 4x4's we inflate them to 40psi thats just wat i was told to do.

Grrrr It really annoyed me when I first had my boggers fitted to rims here, they inflated them to 35psi. Max psi on this tyre is 30!! I know they are probably tough enough to take it but i'm sure that number is on there for a reason...

Website · Post by **murcod** » Sun Sep 14, 2003 11:36 am

taziiy wrote:i fit tyres part time on sat when ever we put new tyres on 4x4's we inflate them to 40psi thats just wat i was told to do.

Just had some tyres fitted to the wife's Excel yesterday (195/50vr15). Usually the tyre places run them too low, this time one side had 34PSI (which is a good pressure for them) BUT the other side was at 38PSI! Work that one out!? :lol:

Post by **hypo** » Sun Sep 14, 2003 11:33 pm

i remember Wendle from the board sayin that he runs his 37" MTR's at something like 26 or 28 PSI :?:

correct me if im rong :?:

how is the tyre wear at this pressure Wendle ??

Post by **Wendle** » Mon Sep 15, 2003 8:38 am

hypolux wrote:i remember Wendle from the board sayin that he runs his 37" MTR's at something like 26 or 28 PSI correct me if im rong

how is the tyre wear at this pressure Wendle ??

yeah, about 30 or a bit less if I can't be fuct waiting for the ARB's wheezing exhalation to fill them..
They are wearing well, have about 10000k on them now and still look newish.. I have some weird front camber issues at the moment to address because of the MQ knuckles, and the MTR's still don't seem to be scrubbing out.. They are a very good all round tyre.

Post by **auto_eng** » Mon Sep 15, 2003 10:36 am

'Recommended' inflation pressures change a lot depending on the type of vehicle and the type of tyre.

Eg. The inflation pressure for a commercial vehicle fitted with LT tyres can be different to the inflation pressure for the same vehicle fitted with passenger car tyres.

Take your vehicle - 900kg (unladen mass, not Gross Vehicle Mass). Tyre placard will always specify an inflation pressure for the vehicle full loaded and sometime (not compulsory) for the vehicle unloaded. If I was to give you a recommended pressure for 'unloaded' driving I would do it like this.

Vehicle weight unloaded - 900 kg
Weight distrubuition - assume 60/40 front/rear (would do this more accurately in reality)

This give a front axle load of 730 kg and a rear axle load of 360 kg. Therefore the corresponding tyre loads will be 365kg and 180kg.

I'm assuming the tyres you have fitted are LT (Light Truck) tyres. I couldn't see your exact tyre listed (sorry don't have time to look further at the moment) but for a 33 x 9.5 R15 LT tyre the loads above are below the lowest listed inflation pressures for that tyre - 740 kg at 25 psi. Because of this I would list your lowest inflation pressure 25 psi.

Rule of thumb is +4psi for hwy use. Remember this is a minimum pressure not a max. You can inflation the tyre as high as you want so long as you don't go over the tyres max inflation pressure. I usually run 35 psi but do a lot of hwy.

The way to work out passenger car tyres if different though.

Post by JK » Mon Sep 15, 2003 12:50 pm

Big thanks auto-eng... I have been looking for some guidelines. Great tech.

Post by **bubs** » Mon Sep 15, 2003 1:19 pm

I run my 33x12.5 MT/Rs at 26psi on the street - this was the pressure which the tyres wears evenily accross the tread

Post by **MQ080** » Mon Sep 15, 2003 2:54 pm

What type of question is this?!?!?!

Only YOU can workout what pressure is best for your truck with the mods you have done. It's all a compromise wear/handling. Manufactures all underquote bigtime the pressure that should be run in their vehicles because a) they want you to feel a soft ride and b) their not the ones paying for the rubber. The latest GTS is a prime example of this, 245/35R19 tyre, Holden wanted to run the tyres below 34psi on their placard, but Bridgestone wouldn’t warrant any of the tyres that were coming back for damaged sidewalls because it was well below their recommended pressure, hence Holden looked elsewhere for a company that would and… that’s why Pirellis can be found on all factory 19” rims

Further to this the load index (usually on our tyres, 3 digits) is a numerical code associated with the maximum load a tyre can carry at that speed indicated by its speed category symbol (the letter bit) under specific conditions. Whereas the speed category symbol indicated the speed at which the tyre can “safely” travel corresponding to its load index under service conditions specified by the tyre manufacture.

In answer to why would different pressures have be found in new tyres???? The tyrefitter stuffed up!

Post by **Buggarr** » Tue Sep 16, 2003 1:16 pm

dosen't running lower pressures on the black stuff put more ware n tare on the steering arms and the like ( isn't it heavyer and cause probs with Power steering)

Post by **MQ080** » Tue Sep 16, 2003 3:58 pm

yes that's true, but they don't care, cause it's far easier too make the steering softer by upping the pump, etc. than caring about rubber... anyway by the time you findout your rack is stuffed it's usally out of warr. anyway

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