Outer Limits 4x4 Board

I've got a Toyota FJ45 with BFGoodrich Mud-Terrain A/T KM 32 x 11.50 R 15 tyres on it. What pressure should I inflate these to for normal daily use on tarmac?

Google 4 psi rule.

SIM79 wrote:Google 4 psi rule.

Cheers. Some sites seem to indicate to use a 6 psi rule for 4wd type tires.

So any recommendations on where to start? 30psi and make adjustments from there?

start with what the manufacturers tyre placard states, and go from there.

joeblow wrote:start with what the manufacturers tyre placard states, and go from there.

The car is 40 years old, there ain't no placard there anymore.
Also, since the wheels and tyres arn't factory, not sure how true the placard, if it was there, would be.

twr7cx wrote:

joeblow wrote:start with what the manufacturers tyre placard states, and go from there.

The car is 40 years old, there ain't no placard there anymore.
Also, since the wheels and tyres arn't factory, not sure how true the placard, if it was there, would be.

I'm with joeblow, but since there's no placard, for a FJ45 and BFGs (I've never heard of Mud-Terrain A/T - must be a new one? ) I'd start with about 32 psi if it's lightly loaded, or 36 if you tend to carry more around.

Note that this is a starting point for the 4psi rule, which I've used for about 7 years and have been happy with the results. I've never heard of a 6 psi version.

twr7cx wrote:

SIM79 wrote:Google 4 psi rule.

Cheers. Some sites seem to indicate to use a 6 psi rule for 4wd type tires.

So any recommendations on where to start? 30psi and make adjustments from there?

30psi?

too low I think I have 31 tires and I think 34-36psi is sort of the right spot, 4wd isn't bouncing around on every bump and stuff.

I did my tyres the other day and run then at abour 40psi, from memory the sidewall says 50psi for loaded with 1000kg.

I find 40psi fine for driving around town but certainly need to lower when going bush.

32 -34 would be fine for that size tire, any more then that and you would start to compromise the grip of the tire to the road (thats with no load in the vehical)

zagan wrote:30psi?

too low I think I have 31 tires and I think 34-36psi is sort of the right spot, 4wd isn't bouncing around on every bump and stuff.

On a Sierra, or an F250? You, and others here, might know what you drive, but some of us have no idea.

Zam wrote:I did my tyres the other day and run then at abour 40psi, from memory the sidewall says 50psi for loaded with 1000kg.

I find 40psi fine for driving around town but certainly need to lower when going bush.

And your vehicle is???

skippy's GQ wrote:32 -34 would be fine for that size tire, any more then that and you would start to compromise the grip of the tire to the road (thats with no load in the vehical)

You may be correct, but how would we know. Could you provide a little more background - please explain how you come to this conclusion.

Tyres are one of the most contentious topics in 4wding - probably because too much "advice" is offered based on completely irrelevant experience. 40psi in an unloaded FJ45 around town? Depending on tyres, I would expect that to be frightening in the wet - if not downright dangerous.

Zam wrote:I did my tyres the other day and run then at abour 40psi, from memory the sidewall says 50psi for loaded with 1000kg.

.

you should always fill based on what the sidewall says. my tires aren't stock. they are 35psi cold max at 1000kg.

the tyre placard only applies to the type of tyre fitted at the factory.

So does your car weigh 4000kg?

What's written on the sidewall is actually the MINIMUM pressure the tyre is rated to carry its MAXIMUM load at.

You can put far more than 35psi in that tyre (in fact the rim will ususally fail before the tyre in an overinflation condition) but it won't be able to carry any more than it could at 35 psi.

Regardless of whether it's a different tyre or not, as has been stated, start with what's on the placard and apply the 4psi rule.

The sidewall of the tyre has no idea what car is sitting above it. The tyre placard has a pretty good idea what car it's attached to, therefore the info on the sidewall is only useful as a guide for total load carrying capacity. It's NOT as useful pressure guide.

Steve.

IMHO the 4psi rule is garbage. Google it and do your own research tho.

I'd also say to check the tyre plac. (maybe someone elses whose is still intact) but not sure if that would refer to radial or cross ply tyres, maybe need to find out if there is a difference.

Or, just try 35psi for a week. If you find its too soft/too hard bump pressure up/down 2 psi or so. You will soon find what suits you best.

Mike

-Scott- wrote:

zagan wrote:30psi?

too low I think I have 31 tires and I think 34-36psi is sort of the right spot, 4wd isn't bouncing around on every bump and stuff.

On a Sierra, or an F250? You, and others here, might know what you drive, but some of us have no idea.

Tyres are one of the most contentious topics in 4wding - probably because too much "advice" is offered based on completely irrelevant experience. 40psi in an unloaded FJ45 around town? Depending on tyres, I would expect that to be frightening in the wet - if not downright dangerous.

I have a terrano they are 2ton.

31x12.5inch wide tires ATM.

30PSI is what you'd put into normal car tires, 4wd tires should be running a bit more PSI at least... after all we're talking about a tire twice the size of a car tire.

I heaven't heard of this 4 PSI rule, I go by the 4wd/car/bike bouncing around the road on small lumps and bumps if it's bounciung around too much you'll need some air in the tires to firm up the ride.

Tires are part of the shock system, so more air equals a more firmer ride though you don't want too much as you can get strange wear patterns etc.

40PSI in 4wd tires don't know I think that's a bit much but low profile and super low profile car tires will be running 40-60PSi otheriwse you'd be busting the rims on every bump and they won't hold up the weight of the car etc.

oldmate wrote:

Zam wrote:I did my tyres the other day and run then at abour 40psi, from memory the sidewall says 50psi for loaded with 1000kg.

.

you should always fill based on what the sidewall says. my tires aren't stock. they are 35psi cold max at 1000kg.

the tyre placard only applies to the type of tyre fitted at the factory.

ahhh....so...so wrong.....

I run 32-33 psi in the Jeep's MTR's, anymore and it bounces all over the place and any less it squirms. Light SWB vehicles feel tyre changes more so than larger land barges. On the beach/mud i run 13/14psi everytime because the lighter weight doesn't cause the rim rolling issues that larger vehicles face, plus it has to go lower to get the same tyre footprint that heavier vehicles create.

I run 40psi in the daily driver's (sedan) 18" low profiles otherwise it rides like crap.

cheers

mike_nofx wrote:IMHO the 4psi rule is garbage. Google it and do your own research tho.

I'd also say to check the tyre plac. (maybe someone elses whose is still intact) but not sure if that would refer to radial or cross ply tyres, maybe need to find out if there is a difference.

Or, just try 35psi for a week. If you find its too soft/too hard bump pressure up/down 2 psi or so. You will soon find what suits you best.

Mike

'why do you think it is garbage?
If your tyre is over or under inflated it will change pressure by more than 4psi indicating you have the wrong pressure in your tyre. So where is the garbage part?

oldmate wrote:

Zam wrote:I did my tyres the other day and run then at abour 40psi, from memory the sidewall says 50psi for loaded with 1000kg.

.

you should always fill based on what the sidewall says. my tires aren't stock. they are 35psi cold max at 1000kg.

the tyre placard only applies to the type of tyre fitted at the factory.

so if I buy a tyre and fit it to my Sierra and then buy the exact same tyre and fit it to a F350 I should inflate them to what the sidewall says? Really, you think that is what I should do? Truly ruly?

well, when LJxtreem ran 325 85 16 Michelin XML's on Critta he definitely definitely ran them at 125 PSI because that's what it said on the sidewall.

truly ruly


Steve

-Scott- wrote:

zagan wrote:30psi?

too low I think I have 31 tires and I think 34-36psi is sort of the right spot, 4wd isn't bouncing around on every bump and stuff.

On a Sierra, or an F250? You, and others here, might know what you drive, but some of us have no idea.

Zam wrote:I did my tyres the other day and run then at abour 40psi, from memory the sidewall says 50psi for loaded with 1000kg.

I find 40psi fine for driving around town but certainly need to lower when going bush.

And your vehicle is???

skippy's GQ wrote:32 -34 would be fine for that size tire, any more then that and you would start to compromise the grip of the tire to the road (thats with no load in the vehical)

You may be correct, but how would we know. Could you provide a little more background - please explain how you come to this conclusion.

Tyres are one of the most contentious topics in 4wding - probably because too much "advice" is offered based on completely irrelevant experience. 40psi in an unloaded FJ45 around town? Depending on tyres, I would expect that to be frightening in the wet - if not downright dangerous.

The best advise u can get about the tires u fit to your vehicle if from the local tire guy who fits them!..

From what i understand about tires and i could be wrong ( but u get that on the big jobs).. but here goes any ways...

When u fit a larger tire to your vehicle u dont need to run as hire pressure, the reason being is that there is a greater volume of air in the tire , the volume of air which is in the tire is what controls the heat and ride of the tire.. if u put too greater pressure in the tire u will have less grip on the road, mainly cause of less tire to road contact, but tis will also depend on the type of tire u are running... at the moment i am runnung Micky Baja Claws 33/12.5/15 on a SWB GQ, and run them at 28 to 30
psi for normal driving and for longer trips i may go to the extreams of 32 psi, but if u are running a highway pattern of a alterrain u may want to run higher as the tire will have more tread to contact the road then forsay a mtr, mud terrain, or a claw...

This may have helped some of just confused the shit out of people... but the best advise i can offer is talk to your tire guy as they will know best, after all it is the safety of u and your family that are riding on your tires...




Skippy
when u put lower profile tires on u need to run greater pressure as the volume of air in the tire is greatly less ...

I wouldn't trust the tire guys fitting them. Every time I have bought tires they just pump them up to 40psi regardless of the vehicle or tire. Even had one lot refuse to sell me a set of 31s for my Sierra saying there is no way they would fit. Finally convinced them that yes they will fit, after they fitted them 2 tires had 50 psi, 1 had 40 and the other had 12

skippy's GQ wrote:
The best advise u can get about the tires u fit to your vehicle if from the local tire guy who fits them!..

Every time I get my tyres fitted I usually get 4 different tyres pressures ranging from 30psi to 45 psi. Use the 4psi rule its simple and it only way to work out the correct tyres pressure for your car.

well maybe u guys should get new tire guys!

skippy's GQ wrote:well maybe u guys should get new tire guys!

Or you could learn to back yourself.

I don't want a tyre guy to tell me what to do. I'll ask for opinions, and consider them, but I'll make my own decision on how to wear out my tyres. That way, I have nobody to blame but myself.

well for the people who dont know as much as u guys do about there own vehicel, tihs is a good place to start... every body has to start learning about there shit some where.

while tyre fitters should know what they are doing sadly 99% don't. how many times have i seen vehicles leave with 40-50 psi in there tyres and a few places i've had arguments with because they would not let them down to factory stated pressures. so if you have a prang because your vehicle could not stop in time because the freakin tyres were too hard who is liable?......go by the placard if there is one, if not ring a dealer, and never use the sidewall pressure as a guide.

skippy's GQ wrote:well for the people who dont know as much as u guys do about there own vehicel, tihs is a good place to start... every body has to start learning about there shit some where.

so what is wrong with learning the 4 psi rule? A rule that will give an inexperienced person the best understanding of how different terrain, loads etc can effect tyre pressure and should be changed accordingly. A tyre person telling you that 40 psi is the right pressure is not telling you the full story and could lead to problems down the track.

grimbo wrote:
so if I buy a tyre and fit it to my Sierra and then buy the exact same tyre and fit it to a F350 I should inflate them to what the sidewall says? Really, you think that is what I should do? Truly ruly?

based on what the side wall says. 35psi at 1000. since my 80 doesn't weight 4 tonne, it's more like 3, i would put a bit less in (i actually run them at 30)

As i said when you modify tyres the placard isn't so accurate anymore. What if my placard said inflate to 75psi cold because it had big skinny truck tyres on it? would you inflate my 35psi tyre to 75 because the placard says so?

You'd have a tyre on the car with an inadequate load rating if that was the case so it's a silly example.

Steve.

Gwagensteve wrote:You'd have a tyre on the car with an inadequate load rating if that was the case so it's a silly example.

Steve.

i disagree, not all tyres are created equal, and that's the point i'm trying to make.

tyre pressure is not proportional to load capacity, after all my mountain bike takes 60 psi, and carrys alot less weight than my 4b does.