under pressure

pass1over

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This isn't mustang related, so i'm not sure if it belongs here or in the Off Topic thread. My apologies if I chose wrong.

I figure you guys know about a lot more than just mustangs. I need some help with my truck.

2003 F550 Super Duty crew car dually, 225/70/19.5. Just recently put tires on it and am a little confused at to what pressures I should be running.


Their Conti HSR/HDR 19.5 on a dually setup. Pull a 3 horse GN with living quarters. Believe the sticker on it says 12k lbs.

OEM tires were General LMT400's

3640lb Single 95psi max
3415lb Dual 95psi max

Current Conti's are

3970lb Single 110psi max
3750lb Dual 110psi max

Sticker from Ford says to put it at

F - 75psi
R - 95psi

Sticker from Utility Bodywerks (who put the bed on) says

F - 75psi
R - 75psi

So, whom do I believe? Why would Utility Bodywerks spec 20psi less in the rears? It's a cab/chassis with a pickup bed on it. Since the Conti's on it can carry more, should I bump up the Ford spec by 10psi or so?
 

Phil

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Proabbly due to load range vs actual load, as under pressurize tires with a heavy load will over heat and blow, I'd be on the higher side when loaded but leave room for the rise in pressure once the tire is at temp.

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redfirepearlgt

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Common sense says to use the specification as set by the tire. IF in doubt ask the tire dealer.
 

pass1over

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Common sense

so rare it's like a damn super power



I think i'm going to take the truck to the scales and see what it weighs, front axle and back axle and go from there.

110psi would be the max to run obviously, but that's not what it should be at. Probably going to end up doing 75/95 like the Ford placard says on the jamb.
 

46addict

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so rare it's like a damn super power

This one is tricky because the the specs on the placard are based on OE tires. The new tires now have a higher load rating and psi limit so at this point it's anyone's guess. If you know the difference between hot & loaded psi versus unloaded cold psi, you could use that figure and subtract from max recommended psi.
 

pass1over

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stock tire was load range F, while the new ones are load range G, hence the higher pressure and load capacity

so your saying (for example, numbers made up) ... if hot/loaded is 82psi, and cold psi is 75, then take the difference, 7psi, and subtract that from the max, giving me 103psi. And that's about what I should run?


this is another one of those ... ask 10 people, get 12 answers kind of things.

Coworker, who ran dealership service dept's and tire/brake stores said they would do 10psi less than max on passenger cars, and 20psi less than max on trucks. Which would put me at 90psi F and R. Close to what Ford originally spec'd for the truck.

I'm just worried that running 75psi on a tire that has a max rating of 110psi will cause it to heat up too much and prematurely fail.
 
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tjm73

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I run the tire pressure that the tire tells me it likes for the load it carries.

I figure this out by running a known pressure and observing the wear pattern on the tire. If the tire pattern looks cupped in the middle, add a few psi and repeat. If it looks like not enough thread is being used on the outer edges of the tire reduce the psi and repeat. I start all this at the vehicle manufactures recommended psi.
 

46addict

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stock tire was load range F, while the new ones are load range G, hence the higher pressure and load capacity

so your saying (for example, numbers made up) ... if hot/loaded is 82psi, and cold psi is 75, then take the difference, 7psi, and subtract that from the max, giving me 103psi. And that's about what I should run?
Exactly. But I'm don't have an engineering background of any sort so take it with a grain of salt. I'm just thinking that if a tire is designed to carry more load, there needs to be extra psi to distribute heat. I figured taking the hot/cold difference and deducting that from the max allowed psi is better than throwing a blanket number.

I'm just worried that running 75psi on a tire that has a max rating of 110psi will cause it to heat up too much and prematurely fail.
I would be worried about this too. Tire pressures on trucks need to be tailored to the work being done and service departments don't have the time to document pressure & load ratings from each tire manufacturer and communicate that with customers. Not to mention when you factor in custom beds/boxes on a chassis cab, the tire pressure also has to be "custom" to match the setup.

I run the tire pressure that the tire tells me it likes for the load it carries.

I figure this out by running a known pressure and observing the wear pattern on the tire. If the tire pattern looks cupped in the middle, add a few psi and repeat. If it looks like not enough thread is being used on the outer edges of the tire reduce the psi and repeat. I start all this at the vehicle manufactures recommended psi.

This is what I do for cars at the track & street but trucks are unfamiliar territory for me. Don't know if truck tires have the same sidewall elasticity as car tires.
 
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pass1over

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the problem is, 19.5's wear like iron. It'll be thousands of miles before any kind of wear pattern is visible in these tires.

Would like to mention to, it's just a normal pickup truck, with a normal pickup truck 8 foot bed on it. There's no tool boxes or utility boxes or anything. Utility Bodywerks is just the name of the company. So there's no extra weight or anything that i'm carrying because of it.

105ptev.jpg
 

stpete

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You're probably better off asking this question, or at least searching at ford-trucks.com. I suspect you won't get a straight forward answer there either. I have the same problem with my Diesel Excursion and different load rated tires. I've played around with the pressures on the current set of tires to find a pressure where it rides as nicely as possible and the tires wear normally. My numbers don't help you though. I do add 15 lbs to the rears when I tow heavy vs. just driving around in it. But that pressure causes the rear axle to hop over bumps with no tow load.
 

tjm73

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This is what I do for cars at the track & street but trucks are unfamiliar territory for me. Don't know if truck tires have the same sidewall elasticity as car tires.

All tires do exactly the same thing regardless of what they are mounted on. They just deflect by differing amounts depending on load. And since these HD truck tires are much harder and don't wear as fast, modify your viewing approach by backing/driving through a strip of water on and otherwise dry patch of pavement and observe the pattern left behind. It will show the same information.
 

pass1over

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You're probably better off asking this question, or at least searching at ford-trucks.com

I have asked over there, as well as on powerstroke.org. I just know that the knowledge on this board isn't limited strictly to s197 mustangs. I figure all the information I can get, the better.

My truck jumps a lot over bumps as it is now, even with the rears at 75. It's like driving an unloaded dump-truck. Kind of, because it is.

modify your viewing approach by backing/driving through a strip of water on and otherwise dry patch of pavement and observe the pattern left behind. It will show the same information.


I have read about this, others use chalk, but I assume water would work just as well.

I know that my truck weighs 9100lbs. Me in it, full front tank, 1/3 full rear tank, no load. With that much weight, is the chalk or water method really going to tell me anything? I'd imagine you could run them almost flat before you'd see cupping or some pressure way higher than max before it would bulge in the center.
 

Racer47

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Short answer - believe Ford, they know best.

I'm not going thru every post and correcting it, but there is more bad info just like the other tire thread.

The higher load rating is good, G vs original F. But that does not mean you need to use 110 psi.

Set tire pressures cold, not hot.

75psi / 95 psi is good for moderate to high load.

If you know for sure you are not going to add a bunch of load, you could easily drop the rear pressure to 60 psi and see if it helps. It probably won't help that much and be sure to air them up with load.

There is no valid water or chalk or whatever footprint thing that will provide any useful info.

pass1over, pm me if you have questions.
 

tjm73

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There is no valid water or chalk or whatever footprint thing that will provide any useful info.

Bullshit. I've done this for years and my Father for decades before me. And had no uneven tire wear problems as a result. All it does is show if some or all of the tread is making contact with the road. When all makes contact, I get relatively even tread wear.

99% of the time the factory recommendation is what is right, but sometimes you have to adjust and this method has not failed me or anyone I know that uses it.
 

Racer47

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Yeah, decades ago with bias tires maybe, today with radials, no.
 

tjm73

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I respect your knowledge. But I disagree with you on this one.

20 Years ago when I had my 90 GT convertible with 275/50R15 BFG T/A's (not drag radials) I had to use this this method to address uneven wear. It worked. Those tires wore best at 28psi. And the 165R15 front's wore (and drove) best at 42psi.
 

bujeezus

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I don't understand why you don't go by the tire manufacturer's recommendation initially. Ford couldn't possibly research every tire made for that and every other vehicle they produce.
 

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I don't understand why you don't go by the tire manufacturer's recommendation initially. Ford couldn't possibly research every tire made for that and every other vehicle they produce.

Ford does not need to research tires, that's why T&RA exists. Ford knows the load the truck was designed for and uses the correct tire size, load rating and psi.

Remember we are talking about 21,000 lbs rolling down the highway at 70+ mph. This is when you follow the recommendation. The safety factor is much lower here than putting big tires on little cars
 

pass1over

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I don't understand why you don't go by the tire manufacturer's recommendation initially. Ford couldn't possibly research every tire made for that and every other vehicle they produce.

tire spec has changed compared to OEM. I could go by what's listed on the door. However, there are 2 recommended pressures, 1 by Ford and the other by the coachworks that put the truck bed on.

I'm more concerned about the tires being under-inflated, heating up, and rupturing. Like what Racer said, I have a lot of weight rolling down the road, i'd like to have it go uneventfully. Oh, but I doubt i'll ever do 70+ in this setup. Truck's pretty tach'd out at 70 with those 4.88 gears, i'll probably be a 65-70'er.

This will be my first time towing this setup long distance. I towed it an hour from our old house to the new house and everything went fine. That was on 10 year old trailer tires.
 

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