I usually run my tires the same pressure as unloaded, but I did start running 40 psi all the time when I got my 275/60R20 Duratracs because they were a little more floppy even unloaded. The extra pressure did get them to feel more like the OEM tire. I wanna say my Duratracs are rated for 51 psi max though.Oh, one thing I forgot to mention that I wanted to talk about.
So, I still have P rated tires. Since I have a marginally rated tire, conventional wisdom says air that puppy up to max, right?
Well, that's what I've always done. However, a while back I stumbled upon a little internet argument on the subject. One fella was vehemently defending NOT doing that. Claiming he had been in the tire industry for many years. Designing and producing.
He said if you pay attention to the inflation tables, P rated tires carry their MAX load at something like 35 or 36 psi (which, SURPRISE, is usually about what's stated on our door sticker to run our tires at). As the chart goes on, the tires pick up no extra capacity at more psi than that. He said that all you will accomplish is ballooning out the tire a bit, wearing the center of the tire more and reducing your contact patch. Even while towing. And effectively make them tow WORSE.
I looked the chart up, and sure enough, the tires carry their max load rating at 36 psi or something like that, even though they have a max psi rating of 44. I've always followed what I thought was the conventional wisdom of airing my tires up to max for towing duties if I was towing heavy and getting close to max specs.
Most of the folks in this discussion wrote the guy off as a kook, but after looking up the inflation tables for myself, seeing that they did indeed carry max weight at only 36 psi, and considering that Ford does indeed have that as the recommended pressure on my door sticker, I thought I'd try it.
I decided to run the front tires at exactly that spec on the door, since they are carrying exactly the same thing they do unloaded. I ran them at 36. I still couldn't completely shake the old habit of airing up more in the rear while towing, so I aired the rear tires to 38. I thought you know what, I'll try it. If I don't like it I can pull over and air them up. I keep my Ryobi 18v compressor under the back seat at all times.
I swear to you that the truck felt a bit more stable and less squirmy this trip. Could it be placebo? I guess it could. But it DEFINITELY didn't feel worse. Last couple of times I went out I had them aired up to max. I won't be doing that again. I'll just run at 36 and 38, and I'm sure I'll be sticking with P rated tires in the future. Unless the size I'm looking at isn't offered in it (I may step up to a 33 next go-round).
So, call me crazy, or believe it, or be skeptical but try it for yourself. That's your decision. But, I SWEAR it was a better feel in the truck. I was a little doubtful myself, but I liked it.
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The same argument is not true for ST tires that you would have on your trailer BTW. Those definitely increase in capacity right up to the maximum pressure.
There is also some weird guidelines around inflating your ST tires if you use them above 65mph. ST tires were originally designed and specificed for 65mph operation but the newer tires are rated as high as 85 mph. This is what Maxxis says:
So the way I read that is that your trailer tires need to have 20% loading overhead in them to handle the weight of your trailer at >75mph. And you need to be able to put an extra 20psi in the tires without over inflating.From 66 to 75 MPH - the tire inflation pressure needs to increase 10 PSI (not to exceed the maximum PSI the tire is rated for) and load should be reduced by 10%
From 76 to 85 MPH - the tire inflation pressure needs to increase 10 PSI (not to exceed the maximum PSI tire is rated for) and load should be reduced by 10%