Excellent way to commit suicide. Tire width has nothing to do with needed pressure. It has just about everything to do with load range and max pressure. At 30psi, your tires are not going to handle half the weight they are rated to at max pressure. Low tire pressure means they will generate heat at highways speeds. This causes blowouts and tread separation. Remember Firestone and the Explorer. Same thing.
His truck probably came with P series tires, which have a recommended pressure of 35 (max for tire). If it came with a LR C tire the recommended would have been 40 (max is 50).
If your tires have a max of 65, I recommend you get them up to at least 40-42.

 

Tire size has everything to do with required pressure. The more tire you have on the ground........the less pressure you need to hold up the same amount of weight.

At max pressure my rear tires will hold 6400 lbs..........yeah, 6400 pounds....that's more than my entire truck weighs. My maximum rear axle rating is 3850. So yeah......the maximum weight my tires will see ever see is about half of their rated capacity.

But anyways, back to the pressure thing. Let's assume my 35's have nearly twice as much rubber on the ground as my stockers. That means that they would require half the pressure to support the same weight. Meaning that at 22.5psi, they'll support the same as my stockers at 45psi.

But, that's a little bit of an exaggeration.......they don't have quite THAT much more rubber on the ground than the stockers. I was just using that as an example. But the principal is the same. Realistically, the tire probably has 50% more tread on ground....so 30-35psi would be about right. The only way to know for sure is to put a line across the tread and see how it wears off.

 

i have 315/70/17's and they are at about 10k miles and showing almost no signs of wear. i am running about 45-50 psi in each tire.

 

Geez... You guys run your PSI real close to the max PSI.

I have 285/75/16's BFG A/Ts mounted on the stock wheels. I have the fronts at 36 and the rears at 34 PSI. Excellent wear thus far.

 

i just left it at what the shop who put them on had it at. tread wear shows good so i have no reason to change it.

 

That's fine.....I'm not arguing against running that much air. If that's what works for your situation, that's great. My point was just that 30-35psi isn't necessarily "under-inflated". It's all relative to tire size and vehicle weight.

 

Then why for the same tire size would there be different recommendations from the manufacturer for different load ratings of tires.
Also in your example of your tires. 6400 might be what they add up to, but at 30 psi, they will be able to handle less than half that. Your vehicle is rated by axle at about 4000lbs per axle. You are well below that rating at 30 and 35.

I run LR E tires that have a max of 80 at 50, with perfect tire wear. LR C with a max of 50 at 40, with perfect tire wear. These on an F150.
For the new TPMS systems where the vehicle came standard with a LR E tire with a max of 80, the TPMS would be set to turn the low pressure light on at 46. The lights are set to come on depending upon LR and max tire pressure, not width.

See this site, there is a chart about a third of the way down.

http://www.nhtsa.dot.gov/cars/rules/...e.html?name=#D

 

Ok, I'm gonna give you an extreme example of how a larger tire requires less pressure.

Are you familiar with industrial tractors......like a rubber tired back-hoe for instance?? Some of the smaller models have front tires similar to a truck tire.....mounted on wheels that are about 17 inches. And, on the rear, they have a larger flotation tire. The recommended pressure for the front tires may be 45psi, but only 20psi for the rear. And even at only 20 psi, the rear could hold more weight than the front ever thought of holding. Why? Because it's a taller, wider tire.

I understand what you're saying about the vehicles with TPMS and load range E tires. That's for vehicles that NEED a LR E tire and NEED 80psi to function properly. An F-150 will never NEED that much pressure. Just because you have a tire that will hold that much pressure doesn't mean that you're required to run it......on a vehicle that doesn't require it.

And I'm not going by width only. Because simply an increase in width doesn't change anything. If you increase your width, that also lowers your section height. So even if you have more tread on the ground side to side, you have less tread on the ground front to rear. The contact area stays about the same and the required pressure stays the same.

Ok, I was going to try to explain this a little better...........but I'm just gonna cut it short and give you one more example. There's a 2000 Ranger sitting outside my window with 285/75-16 Load Range E tires, and only 25psi in the rears. Because, with any more pressure, only about a 3 inch wide strip of the tire actually touches the road. I had another Ranger with 33x14.50-15 Super Swamper SSR's. And even with only 20psi I couldn't get the outside of the tire to touch the pavement.

So if you like running alot of air pressure and have had good results, that's great. I like to run my tires with less air........when it's required. If I'm gonna be towing something heavy, I'll air my tires up. But otherwise, I want the full width of my rear tires on the pavement.....not just the strip down the middle. I'm not encouraging anyone to let air out of their tires.......because, yes, it CAN be very dangerous in certain circumstances. All I was trying to do is make a point that having only 35psi in a set of larger than stock BFG's wouldn't cause any irregular tire wear after only 500 miles.

 

Tractor tires have nothing to do with auto tires. Not because of their height and width. They are low pressure tires and designed for that. They are also designed to hold water for weight. They are also not designed for high speed.
Nowhere did I state you need 80 psi on a LR E max80 tire for an F150. 50 does just fine. 35 would be dangerous.
If you want to talk about high pressure tires, look at a tractor trailer tire. Large tire and meant to have a max of around 105.