General Tire Pressure Question Thread

[SupaSwope]

So I had a general question on If I should air down my tires a few psi over the recommended tire pressure when driving on hot days and or long trips when your psi rises some.

While trying to look threw some old threads I found this post.

Guys, guys, guys, stop talking about apples and oranges.

Different tires need different pressures. Way back when tires used to come with a 32psi max rating. Years ago that became 35psi, and lately tires come often with a 44psi max rating. None of these tires should be run at the exact same pressure.

Any tire should be run at some pressure within say, 8-10psi of the maximum rating. Anotherwords, a 44psi tire should never be run with less than 34-36psi, etc.

For maximum handling(safety), and gas mileage, set the pressure closer to the maximum. Lower pressures ride softer, but they create more heat(blow out), handle more poorly, and hurt gas mileage. Unless you have a 50/50 weight distribution, you should run a couple psi more in the front. This improves handling, and tire wear.

I usually have 44psi tires, and I use about 37-40psi in the front, and 35-37psi in the rear. The important factor is how far from the tire rating do you set the pressure.
Ignore any door jamb or manual psi listing. Those are for the stock tires for the smoothest ride(to sell you the car), not for longevity or safety.

If you look around you will see that there are tires now with ratings over 50psi. Large truck tires are usually rated over 60psi. Always start by checking the rating on the tire. Good luck all,
DW

Now I am still curious If I should air down my tires when they go up a few psi but I was running my 44 max psi tires at 26. The recommend psi on the door panel. I thought I was doing everything right but what should I be running my tires at? 30-35 with the front higher then the rear for traction? I mean it makes sense?

I am looking for a general discussion here and what are you/should you be using?

 

[lobo411]

IIRC, Ford doesn't use the door panel number anymore. I used to take mine to Costco and have to ask them to set mine at 35 psi from the recommended 26. A few years ago, Costco started setting them to 32 by default. The guy said that that's what Ford recommended.

 

[J_C]

It is more false than true that higher PSI rated tires should have higher PSI in them due to that. The correct PSI in any tire depends on the load on it. Higher PSI rated tires are merely engineered capable of a higher load and the PSI it takes to support that in any particular tire size. They should be at the same PSI as any other tire of the same size with the same load.

Switch from stock tires to a different size and you may then change the necessary PSI to maintain the proper, even tread contact with the road.

No you should not air down your tires on hot days or long trips. That can actually make them run hotter. I and some others have speculated that Ford chose the low 26 PSI for a softer ride, not for the normal wear or load handling concerns and for that reason I suggest closer to 30PSI, or 35 PSI if you have a heavy load, unless you have larger than stock tires.

Look at your tread wear and that will guide you to an adjustment of higher or lower pressure. You can also take a white crayon and make a mark across the tread to see if it is evenly scrubbed off the bottom, driving straight for a brief period.

http://www.fourwheeler.com/how-to/wheels-tires/154-0704-tire-pressure-science/

 

[SupaSwope]

Thanks for all the help

 

[CDW6212R]

The goal is to achieve even tread wear, and safety and good handling, ride etc. The higher pressure tires don't wear the tread the same as a lower(32psi MAX) tire, at any given pressure. The 44psi tires will wear out the edges much faster if you run them at 26psi, or 32psi, versus the ideal amount. Finding that ideal amount is different for every vehicle and each different load. You cannot copy a different car's ideal pressures, or mine, or any "expert's" opinion. Yo should do as some suggest, start at a certain level below the MAX, more on the heavy vehicle end, and adjust it often by testing. Testing can be with chalk, as mentioned, which is a vague quick method that can get you in the proper range. It takes hundreds of miles and careful monitoring to zero in on the best pressures.

I go up in pressure to wear the center more, down to improve the ride if it's too harsh etc. You will know if you are close based on tire wear, checked very often early on, and less and less after months of driving. I'm at the 37/34psi range with my mail truck, a 4500lbs V8 Explorer. That's working well for my truck, my driving, for my usage, with the Michelin tires I have. The tires are wearing fast, but evenly, my usage eats tires. For a commuting vehicle you could get 50k miles with these tires, but my route will be lucky to net me 20k miles.

 

[jadatis]

The pressure-advice is cold measured.
And cold means when, inside tire temp = outside tire temp , that simple .
Cold advice is given for 18/20degrC/65/68degrF, and if colder outside pressure becomes lower, so more heatproduction , but also cooling down of tire is better because of the larger temperature differences between rubber and in/outside-tire air.
The deflections of every segment of the tire about 10 times a second, when driving about 80km/50m/h, produce heat of rubber, and warmes up the tire inside air, wich highens up the pressure of tire.

So on a hot day the cold measured pressure is higher because inside tire temp = outside tire temp is higher.
But the tires then need that higher cold pressure to give it lesser deflection, so lesser heatproduction.
This because the cooling down of tire rubber is also lesser because of the smaller temperature differences between rubber and the rest.

So dont blead air from tires on a hot day, to give it the same pressure cold measured as the advice pressure.

Then about the pressures given on sidewall of tires.

The maximum load is calculated by tiremaker(-organisations) for a fixed pressure wich is called the reference-pressure in the official European formula I once got hold of and went running with.
Will call it AT-pressure, because on LT and trucktires its given that way on sidewall.
for American system tires ( P-tires , made in Standard load and XL/reinforced/Extraload) this AT-pressure is next standard.
SL AT 35 psi/2.4 bar. ( EUR system AT2.5bar/36psi with exeptions to lower , fi the 32 psi given in beginning post).

XL// AT 41 psi /2.8 bar ( Eur system AT 2.9 bar/42 psi with also exeptions allowed).

On the sidewall is only P-tires given the maximum allowed cold pressure by the tiremaker, of 44 to 51 psi for SL and XL sometimes up to 60 psi. The space between AT and maxcold is used to highen up the AT-pressure for higher speed , with a system depending on speedcode of tire.
Also for alighnment ( camber angle more then 2 degrees) a system for highening up the At-pressure.

Before 2000 I think, the AT pressure was given on sidewall of SL and XL tires of that 35 psi or even 32 psi.
Later they only give maximum cold pressure.

So warm pressure , the pressure it becomes when the tire inside heats up by the deflections of tire at a sertain speed, can even be higher then the 51 psi maximum cold pressure, and that is no problemm.

Once read that tires are tested to can stand a pressure of 2 to 3 times the AT-pressure.
This would mean that your 35 psi AT pressure tire , can rise to 70 or even 105 psi , without blowing or damaging .
Ofcource the ride would be verry hard then because then deflection becomes about zero .
If you would fill the tires at freesing point of water ( 32 degr F ) with 1.4 times the AT-pressure , and by severe braking , when descending the mountains, the tire inside would reach boiling point of water ( at 1013 mb) , the pressure in tire would rise to about 2 times the At-pressure, so still not blowing or damaging tire, by the pressure alone.

Hope this all will give you a better idea of what you can do , must sertainly dont do , at different ambiënt temperatures.

Greatings from a "Pigheaded Dutch Selfdeclared Tirepressure-specialist"
( ja dat is) Peter

 

[CDW6212R]

Hi Peter, I wondered about your language and that some of it might be posted from some manual etc. I understand what you said, you do fine at writing in English. Most of that is basic physics concepts, which help me a lot in learning about new things.

Tire pressures are a massive subject simply by the varying conditions tires must endure, and everyone drives differently. It's all about tread wear, and how you figure out what pressures are best for your vehicle. Remember that this applies to each vehicle separately, meaning it's different for each car you drive.

 

[jadatis]

For the users of cars, things like treathwear and the conditions the tires must endure, etc, are important,but for the car-and tiremakers only counts that their tires stay save, and wont get damaged by driving with the speeds and loads on them.
And the maximum load of a tire is calculated for the AT-presssure and the reference-speed of tire (up to Q speedrated maximum speed of tire). So when you have exactly maximum load on a tire and AT-pressure in tire, you can drive up to reference-speed ( mostly above Q so 160km/99m/h), and then no part of rubber of tire gets above a sertain temperature at wich it hardens and crackes in next bendings of rubber by the deflections.

for lower loads the pressure is so calculated, that the deflection stays the same , so heatproduction the same.
You have to use a pressure then the same or higher then calculated.
Above that you can play for better handling or less wear of treath, sideward stifness,etc.
But if you go to high comfort and gripp gets lost, but no danger to tire.
I determined by reactions about bumping , that if real weight on tire is 85% or more then the weight the pressure is calculated for , for 160km/99m/h, with the best formula to laws of nature, that then comfort and gripp is still acceptable.
Verry discussable, and comfort is subjective, and dont count for low H/W division tires .

 

[Rhett]

Also for alighnment ( camber angle more then 2 degrees) a system for highening up the At-pressure.

My alignment guy mentioned to me, something about running higher psi in my front tires due to the camber angle, which was around 2 degrees, (not using Moog K80109 bushings), I think he said something like 40-42 psi, which is higher than I'm used to (35 psi cold usually)

 

[429CJ-3X2]

I posted this in the Sport Trac sub forum earlier this week. Tires are Firestone Destination A/T, 255-70-16 stock rims.

I checked the air in the tires of my '01 ST. They were all about 30 psi, so I pumped them up to about 35. Not a huge deal, right? WRONG! This truck has just a little slop in the steering, I think due to wear in the steering shaft itself. At 30 psi, both the steering and ride feel soft, and it feels like it wallows a bit on the road. At 35 psi, the ride and steering are much firmer. Drives like a different truck.

 

[jadatis]

My alignment guy mentioned to me, something about running higher psi in my front tires due to the camber angle, which was around 2 degrees, (not using Moog K80109 bushings), I think he said something like 40-42 psi, which is higher than I'm used to (35 psi cold usually)

When camberangle is 2:30 degrees ( 2.5 decimal) the calculated pressure should be only 3.22% added .
3 degr Camberangle 6.62% extra
4 degr CA 14.08% this all to the official calculation .
so your 35 going to 40 is even 14.28% adding , would be enaugh for 4 degrees camber-angle, wich in the sheet is adviced as maximum to use.

But because the official system says so , does not mean that , to laws of nature , this system is right.
For low Hight/widht division tires , I already determined that more would be needed.
And in the system they also give for low H/W tires a maximum of 2 degr camber angle .
Also for high speeds above 270km/??m/h no more then 3 degr CA is adviced.
its all about to the trapezium shaped surface on the ground is smaller then a rectangular shaped , when no camber angle.

 

[Gary Powell]

IIRC, Ford doesn't use the door panel number anymore. I used to take mine to Costco and have to ask them to set mine at 35 psi from the recommended 26. A few years ago, Costco started setting them to 32 by default. The guy said that that's what Ford recommended.

I've been having alignment problems, pulling to the right, and I had it aligned and it drove alright and started doing it again. So I figured I would put 40psi on my left tire and 36psi on my right, and still no help.

 

[SupaSwope]

I raised my fronts to 35 and rears to 32. I feel good about it honestly.

Also was gonna ask. What is the normal operating temperature for our size tires 235-255 range. Especially on days were you traveling cross county and driving for 8 or more hours.

I was gonna order one of those tire pressure monitoring system you plug into your car and put on the fill valves that have a temperature gauge.

 

[CDW6212R]

I have one of those $75 TPMS kits, I'll let you know how it does soon, after I change my radio bezel and get the cigarette outlet working again. It's only five components and the display is fairly small(takes up little space).

 

[SupaSwope]

Sounds Awesome. I need to replace my cigarette lighter as well. I just don't know if its the socket or what, I don't know which one to buy either.

 

[CDW6212R]

My previous owner tore out the plastic of the bezel(common issue), but they left the socket hanging out, which doesn't work. I'll have to figure out what they did to it, the fuse is good but it's a small 7.5 amp circuit.

 

[J_C]

^ ?? Lighter outlet circuit shouldn't be only 7.5A. My '98 fuse diagram shows fuse #17, 30A.

Once when I took that panel out it tore the plastic stud(s) off the back of one or two corners so I set them back in with epoxy after roughing up the surrounding area with sandpaper to get more adhesion, walling off a little fort area with tape to pool the epoxy deep enough. That must've been a good 12+ years ago, still holding on.

 

[CDW6212R]

^ ?? Lighter outlet circuit shouldn't be only 7.5A. My '98 fuse diagram shows fuse #17, 30A.

Once when I took that panel out it tore the plastic stud(s) off the back of one or two corners so I set them back in with epoxy after roughing up the surrounding area with sandpaper to get more adhesion, walling off a little fort area with tape to pool the epoxy deep enough. That must've been a good 12+ years ago, still holding on.

The models with a full console have the big fused outlet on the right side of the console, and the dash outlet is the small 7.5 amp circuit. I'm not sure what the short console models would use for the fuses, the manuals may be wrong or confused. I found out how weak the radio power outlet was when I tried to run my air compressor. It would blow the fuse after a short time. When I saw the tiny red fuse, I stopped using that outlet except for charging a phone.

 

[J_C]

The models with a full console have the big fused outlet on the right side of the console, and the dash outlet is the small 7.5 amp circuit. I'm not sure what the short console models would use for the fuses, the manuals may be wrong or confused. I found out how weak the radio power outlet was when I tried to run my air compressor. It would blow the fuse after a short time. When I saw the tiny red fuse, I stopped using that outlet except for charging a phone.

I'm not sure what you mean. I have what I thought was the full console, goes all the way back to the armrest, which does have a 2nd lighter outlet on the right side of it, but thought that is a lower powered outlet. It doesn't really look heat resistant like you couldn't use it with a lighter, though I suppose that doesn't necessarily make it lower powered, if it's on the same circuit, might even be capable of more power in real life if the radio outlet has corroded from heat or fouled tobacco/ash residue.

I have no 2nd fuse in my fuse diagram, only the one #17, though in the manual, page 182 it is listed as 25A instead of 30A. I did notice that this value increased, back in '96 it was only 15A.

I suspect yours has the wrong fuse in it.

 

[CDW6212R]

It's been since 2006 that I rewired my 99 truck. That's when I was looking at the fuse/circuit sizes, I used the seat heater circuit(20a light blue/white) for my sub amp circuit. I didn't use the console power outlet at all since left out the console. But I recall that the console outlet is either 25a or 30a, my 98 Mercury is the one I discovered the dash outlet being the little red fuse circuit. I'd expect the 99 and 98 I have also use the same red fuse for the dash outlet. I'm not sure how they are labeled in the owner's manual, 2005 was the last time I looked at that closely.