11screw50

OK, so one of the things I stumbled upon while doing some tire research related to tire pressure and load carrying capacity. While not quite exact, the rule of thumb seems to be that if you take the maximum tire pressure and the maximum weight carrying capacity of the tire, the ratio of actual pressure to max pressure is the same as the ratio of actual weight capacity to max weight capacity.

So if you have a load range 'E' tire with a max weight capacity of 3415 lb (at 80psi) and you run it at 40 psi, the weight limit is 1707.5 lb (half of max).

Where it gets interesting is when comparing across load ranges, the higher the load range, the more pressure you need to carry a given load. A P rated tire of the same tire noted above could have a capacity of 2600 lb at 44 psi (though P tires are a little different because while max is 44psi usually, their weight capacity is rated at a lower pressure for some reason). To get that same 2600 lb capacity on the LRE tire you would need 2600/3415*80=60.9 psi.

Just something most people do not think about (and a good reason to adjust your tire pressure up before towing (unless you are one of those people who run at max pressure all the time...and if you are, you must like the rough ride).)

usernamenotvalid

So if you have a load range 'E' tire with a max weight capacity of 3415 lb (at 80psi) and you run it at 40 psi, the weight limit is 1707.5 lb (half of max).

Theoretically , yes.

I just went thru all this .......E rated tire with max load /pressure of 80 psi.

l searched for data on load vs pressure and only found that Goodyear had an outdated matrix and didn't show 225's and some other ST tires.

After I called the tire manufacturer (Hercules) it took me two phone calls and two e-mails to get an answer to my question.....he sent me a portion of the Tire and Wheel Association Handbook and its a PDF and I can't seem to "highlight and send". Basically it shows that tire pressure rise correlates to load capacity with a max of 80 psi.
He did say, when he sent this data, to not go below 65 psi in the tires. (ST225/ 15 E range with 6" wheels. Wheels rated for 2600 pounds and axles rated 5500#.

I am way over tired as with two axles, 2500 pounds load per tire will more than cover the gross weight of the trailer (8 K). Thinking that I can use 45 or 50 psi so giving the trailer as smoother ride.....but not so on using lower pressure.

TIRE HEAT is a big factor. So it's 65 psi and we will see this weekend how it "drives".

U would think that there was more data availiable for ST tires.....a number of folks have had problems with tires on their trailers. it might be from under inflation. Hope this helps you..

11screw50

OK, I guess I should have clarified, I meant tires on the truck. It is my understanding that trailer tires are run at max pressure...

Toyo has a good pdf that explains going from a P-rated tire to an LT-'E' tire. In their example, the nameplate on the truck says 35 psi and they show that with the LT you would need 45psi.

Cover Dog

This was posted by smokeywren several weeks ago....

You need to find a load/inflation table for your exact construction and size of tire.

The HD Payload Package comes with LT245/75R17E tires. Brand doesn't matter as long as they are LT245/75R17E. So here is the load/inflation table reprinted by Toyo from the original Tire&Rim Assn tables:

LT245/75R17E Single:
PSI. Max Weight
35 . 1770
40 . 1945
45 . 2110
50 . 2270
55 . 2430
60 . 2595
65 . 2755
70 . 2900
75 . 3050
80 . 3195

If you look on the tire, the only load inflation number you will see is 3195 pounds @ 80 PSI. But if you do enough digging, you will find the above table.

It says that at 35 PSI, that tire has a weight capacity of 1,770 pounds when mounted on an axle that has single tires (not duals).

You want to have the right amount of air pressure in the tire to support the load you will be hauling on that axle. The minimum is the amount shown in the above table for the weight on the tire. Over-inflation by 5 to 10 PSI is okay, but under-inflation is never okay. So on my F-150 when towing I have 3840 pounds on the rear axle, or 1920 pounds on each tire. So per that table I need at least 40 PSI in those LT245/75R17E tires when towing. If I prefer a little fudge factor, I could run 45 PSI and have a lot more tire weight capacity than I need. So those tires that can hold as much as 80 PSI would be overkill for my F-150.

But if I had those tires on an F-150 with the HD payload pkg, and my wet and loaded rear axle weight was 4,250 or 2,125 per tire, how much PSI would I need? 45 is not enough, but 50 is plenty. So with 4,250 pounds on the rear axle, I should run 50 PSI in the rear tires. That's a lot less than the 60 PSI on the door or 80 PSI max on the tire sidewall, but the guru engineers at the Tire&Rim Assn say that's all I need. More than 50 PSI and I'll have a rougher ride, and the center of the tread will wear faster. But less than 50 PSI won't give me enough weight capacity to haul my load, so the tire will overheat and probably blow out.

The lawyers have the tire companies scared to death of lawsuits, so most won't provide needed load/inflation info easily. They believe the average Joe is to stupid to understand and apply a load/inflation table. So they tell you to follow the vehicle manufacturer's guidelines (then you cannot sue them, you have to sue Ford if you have tire problems). The info is available, but you have to dig it out.

The source is the Tire and Rim Assn's annual report to members. But membership is expensive so you and I aren't members. Michelin truck website has some of the data for bigger trucks and motorhomes, but not for light trucks. BFGoodrich is part of Michelin, so you probably won't find load/inflation tables or BFGoodrich tires either. You probably can't find it for P-series or LT tires on Bridgestone or Goodyear websites . Toyo still makes it available, so that is where I found the above table. But you better hurry before Toyo realizes that they are a lawsuit target by making those tables available to Average Joe.
http://toyotires.com/tires-101/load-...flation-tables
______________

hunterv

So how much would you increase the pressure in the front tires? the same as the back?

BassAckwards

On our trucks you generally keep the pressure in the front the same as the rear. That's just what is suggested by the manufacturer.

And as for P-metric air pressure vs LT-metric air pressure it all depends on what the load index of your tire is as well when you swap an LT tire on our truck.

My old 275/70R18 DuraTracs were load index 125 and to hold equivalent weight to my P-Metric I needed about 45psi because they were Load Range E1

My new 325/60R18 Cooper STT are Load Range E2 and only need 35psi to hold equivalent weight to my stock P-metric and they are load index 124 and actually hold less max weight than my DuraTracs.

My point being that there is no set air pressure when swapping to LT it's all relative to the tire you are putting on.

And truth be know any LRE tire will hold more weight at 35psi than our trucks are legally able to haul.

Cover Dog

My 2013 F150 Lariat 4x4 SCrew w/157" WB with the Max Tow & HD Payload Pkg my door sticker for my "E" rated tires says to runs the front at 55psi and the rear at 60psi.

11screw50

Not according to Toyo they won't. But I agree, it is relative to the tire.

usernamenotvalid

Ford F150 Forum - Community of Ford Truck Fans > Ford F150 Forums > Towing/ Hauling/ Plowing

So I guess 11screw50 would have be talking about trailer tires as this was the tow section.