I would be extremely cautious of a mechanic who gave you this type of adivice. The tire manufacturer knows what the maximum capacity of the tire is and what it will handle. The vehicle manufacturer knows what the vehicle requires to haul its butt down the road. This isn't a hard and fast rule and obviously tire size changes and changing loads require adjusting to different pressures.

Look at it this way a Ranger running a 33 X 12.50 in no way needs the same load capacity (tire inflation pressure) as a loaded SuperCrew running the same tire/size.

While under inflation can be dangerous as it builds excessive heat, over inflation dramatically decreases how long the tire is going to last and how it handles. I would trust that door sticker more often than the maximum load pressure listed on the tire (unless hauling a heavy load). This also gets a little "iffy" when comparing P-Metric tires and LT tires. P-Metric tires ratings are diminished in an LT setting. When a tire is made the manufacturer has no idea what so ever the type of vehicle it will end up on, they just know what it is capable of hauling when inflated to a specific pressure. When your truck is made Ford knows exactly what it takes to move it safely down the road, load capacity is load capacity regardless of tire design (again watch out here when making the conversion between P-Metric and LT tires). Just my 2 cents.

 

I sincerely thank you for your input. I have searched a fair amount on the wonder worldwide web and have yet to find more than a couple of reasonable sources that gave any theory behind the numbers.

But I'm curious then...is that why Ford and Firestone paired up and opted for underinflated tires on SUVs so the ride would be smoother? Doesn't sound like they had the specific load carying capability in mind when they when through that nightmare.

My sticker says to inflate my fronts to 29...and rear to 31. With that inflation, my tires look almost flat. There is no way I'm going to go with those pressures. I'm not an engineer for Ford or BFG, but I'd place money that these would be seriously underinflated.

Anyone elses thoughts out there...I am really curious to learn more about how to determine correct tire inflations for their trucks.

Thanks...Scott

 

The Ford/Firestone fiasco wasn't due to the fact that the reccomended pressures were too low for the vehicle. The problem was that the tires were being run under inflated (the average driver doesn't check pressure monthly). Obviously this is easy to do and the majority of tires on American highways are low on air pressure.

The design of these particular Firestone tires couldn't take the additional heat generated while running low and self destructed. Ford's fault in this problem is that they had vehicles with a relatively high center of gravity running around with inferior tires. This made the situation ideal for a rolled vehicle. The replacement tires used in resolving this problem were still using the original suggested inflation pressures but Ford became much more emphatic about adhering to the recommended pressure.

By the way Firestone has had this problem repeatedly throughout their history. Back in the late 70's and early 80's they had two tires that exhibited similar problems. These tires were the Firestone Radial 500 and the Radial 721. These two tires just about bankrupt Firestone and actually allowed Bridgestone to come in and buy them out. I better quit now, I think I'm starting to sound like a nerd. By the way I used to run two districts (18) of tires stores I've seen most of these problems first hand.

 

GearHead...Thanks for the clarification regarding the tire fiasco. I got my BFGs through the recall they had.

Bridgestone is made by Firestone (or visa versa)???

Since you have a good grip on tires...is the Bridgestone Dueler A/T REVO and the Dueler HP decent tires? I'm thinking of putting the REVOs on when my BFGs wear out.

But back to tire pressure again. The only load I carry is a cap on the bed (approx 200#) and misc. gear (approx. 100#) so there really isn't that much extra weight in there. Is there a formula or guideline to help determine the tire pressure depending on the amount of weight carried in the truck?

Thanks again...

 

Rottie,

You're more than welcome. Bridgestone owns Firestone. Bridgestone makes a very good tire given it's relatively modest price. I have some personal feelings on their products that by no means makes them less of a tire. Most of their tires do not use full depth cuts on their siping. In effect this makes the tire a much better tire in it's first half of life than towards the end of it's life. This isn't just because you have less tread depth. I know this almost seems like a DUH, but the reality is that some of their siping cuts begin to dissapear within the first 30% of the tires life. A tire like a Michelin for instance uses siping that is cut all the way to the wear bars. So in effect you have the same number of sipes at the end of the tires life as it had in the beginning even though at a reduced tread depth. That said, Stones are very good tires.

We used to use a rule of thumb (I can't show you documentaion on this anywhere) but for every additional 500 lbs. of load, increase the air pressure 5 lbs. up to within 5 lbs. of the maximum rating on the tire. Adjusting this level down as the load is decreased.

Of course you can figure out a more precise pressure by knowing the exact weight of the vehicle, loaded or unloaded and take the max load capacity of the tire at it's limit and adjusting downward proportionally. If you ever found that you had to adjust upward using this description you either have a tire that is underated for the load you are hauling or the truck is overloaded. Just remember you are doing this on 4 corners and it's not exactly even weight distribution front to rear. Once again Ford has supposedly figured this out on the door sticker.

One other thing, I noticed you mentioned that your tires "looked" low. This is not necessarily uncommon on modern radial tires. Don't let the look of the bellied out sidewall fool you and become your determining inflation factor.

 

Good info on the Bridgestones regarding the siping and tread wear. I'm still in a toss-up on returning to the BFGs or going with something new as in the REVOs.

Also appreciate the rule of thumb onload -vs- increased tire pressure.

I'd like to send you a couple on me...but this will have to do

Thanks again...

 

The door sticker is for a balance of ride , handling, and load.

What your truck is rated to hold requires no addtional tire pressure. If you are going to overload your truck then you could add a few psi.

Again the max psi rating on a tire sidewall is MAX COLD PSI so if you pump your tires up to that number and then run them on the road the will go way way over that number. Never ever ever fill your tires to what the max reading on the sidewall says. Ever notice that the max psi on the sidewall is always a lot higher than what the door sticker says? (Which is prolly what always gets this debate started in the first place!) It is higher for the great fluctuations that your tires go through while being used.

Lets say the door sticker says 28/front and 30/rear the max reading on the tire should be 40-45 because when you drive a front tire with 28psi on a hot sunny summer day the combination of load, brake heat, road heat, resistance heat, engine heat, direct sun heat, etc can crank the 28psi upto 30-35psi now imagine if you follow the urban legend of pumping your tires up to the max psi rating on the side wall and then apply the above heat!

I can't say this enough the max rating on the side wall is just that it is max!! It is not max what you can put in and be safe it is max what the tire can handle. Also remember it doesn't say on the tire "inflate to..." it say MAX PSI.

Sorry for the soap box rant-

 

Here is a link to your question on the goodyear web site and if you scroll down the page to the next question there is a link to an inflation vs/load manual

http://www.goodyeartires.com/faqs/Inflation.html

 

HCMQ,

Thank you, that link says what I tried to in 3 way to long posts.

 

"P" Passenger tires = 35psi max cold
"LT" Light Truck Load C tires = 50psi max cold
"LT" Light Truck Load D tires = 65psi max cold
"LT" Light Truck Load E tires = 80psi max cold

Tire pressure is dependant on what tires you have on your truck right now, not what the door sticker says. With-in 10% of max is about right. When I had P tires on my f150 I kept max 35psi all around, but when I loaded up, it still got alittle mushy.

Now I have LT load C tires. When the truck is empty I run 45psi front, about 42psi rear for a better ride. When the truck is loaded up I put 50psi all around. JMHO

 

Sag,

You are asking for trouble. Max is Max not what is safe or reccomended. Your theory is like saying, "Well the owners manual says to use 6 quarts of oil max but I know better and I will put in 10 because that is what it will hold, because more is always better"

I guess I just don't understand why this is so hard to understand... Max is Max it is NOT what you can pump the tires up to. It is the most pressure the tires can handle. So if you put in 45psi and the sidewall says max is 45 psi then you tow your psi goes up to 55-65 is that ok? but wait the tire says that 45 is max?!?!

I bet the centers of your tires wear out first on all the past vehicles you have had. And your truck prolly gets lousy traction in the rain.



Oh well to each his own. As I have always said on this site do what makes you feel comfortable.

 

hcmq,

I am not an expert, but from my understanding the max rating is the max safe "cold" pressure. So if the tire is rated at 35 psi you can pump it up to 35 psi. It is not the maximum operating pressure the tire can withstand.

I have always run 35 psi in P rated tires and get excellent wear from the tires.

 

FX,

If you check the link that I posted above it clearly states that psi on the side wall is in fact the Max operating pressure not the max cold psi.

 

to add to the confusion:

http://www.nhtsa.gov/cars/rules/ruli...ty.html#secV_A

it appears that there is a lot of confusion in the world about the inflation numbers on the sidewall. I have just spent the last few hours searching sites for an answer.

All of the "Know it all" sites say the psi on the sidewall is a max cold inflation pressure. And all of the manufacturer sites say it is a max overall inflation pressure.

no wonder the nhtsa has gotten involved! It says that they are proposing to remove the side wall psi rating all together!

 

Ok,

I just checked on my tires and on the door jamb to see what Ishould be running at. On the door jamb it said 30 in the front and 32 in the rear max cold psi. On the tires it says MAX pressure 44psi.

I checked the pressure on my tires and 3 of the read right around the 31-32psi mark. On the right front it read around 28psi. My tires are stock 255/70/16. What would be a safe pressure to put into the tires?